Module: support defining const and use const variables inside of function (#9773)

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# Description
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Relative: #8248 

After this pr, user can define const variable inside a module.

![image](https://github.com/nushell/nushell/assets/22256154/e3e03e56-c4b5-4144-a944-d1b20bec1cbd)

And user can export const variables, the following screenshot shows how
it works (it follows
https://github.com/nushell/nushell/issues/8248#issuecomment-1637442612):

![image](https://github.com/nushell/nushell/assets/22256154/b2c14760-3f27-41cc-af77-af70a4367f2a)

## About the change
1. To make module support const, we need to change `parse_module_block`
to support `const` keyword.
2. To suport export `const`, we need to make module tracking variables,
so we add `variables` attribute to `Module`
3. During eval, the const variable may not exists in `stack`, because we
don't eval `const` when we define a module, so we need to find variables
which are already registered in `engine_state`

## One more thing to note about the const value.
Consider the following code
```
module foo { const b = 3; export def bar [] { $b } }
use foo bar
const b = 4;
bar
```
The result will be 3 (which is defined in module) rather than 4. I think
it's expected behavior.

It's something like [dynamic
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding-Tips.html)
vs [lexical
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Lexical-Binding.html)
in lisp like language, and lexical binding should be right behavior
which generates more predicable result, and it doesn't introduce really
subtle bugs in nushell code.

What if user want dynamic-binding?(For example: the example code returns
`4`)
There is no way to do this, user should consider passing the value as
argument to custom command rather than const.

## TODO
- [X] adding tests for the feature.
- [X] support export const out of module to use.

# User-Facing Changes
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crates/nu-parser/src/parse_keywords.rs

@@ -940,9 +940,8 @@ pub fn parse_export_in_block(
     let full_name = if lite_command.parts.len() > 1 {
         let sub = working_set.get_span_contents(lite_command.parts[1]);
         match sub {
-            b"alias" | b"def" | b"def-env" | b"extern" | b"extern-wrapped" | b"use" | b"module" => {
-                [b"export ", sub].concat()
-            }
+            b"alias" | b"def" | b"def-env" | b"extern" | b"extern-wrapped" | b"use" | b"module"
+            | b"const" => [b"export ", sub].concat(),
             _ => b"export".to_vec(),
         }
     } else {
@@ -1000,6 +999,7 @@ pub fn parse_export_in_block(
     match full_name.as_slice() {
         b"export alias" => parse_alias(working_set, lite_command, None),
         b"export def" | b"export def-env" => parse_def(working_set, lite_command, None),
+        b"export const" => parse_const(working_set, &lite_command.parts[1..]),
         b"export use" => {
             let (pipeline, _) = parse_use(working_set, &lite_command.parts);
             pipeline
@@ -1400,6 +1400,59 @@ pub fn parse_export_in_module(
 
                 result
             }
+            b"const" => {
+                let pipeline = parse_const(working_set, &spans[1..]);
+                let export_def_decl_id = if let Some(id) = working_set.find_decl(b"export const") {
+                    id
+                } else {
+                    working_set.error(ParseError::InternalError(
+                        "missing 'export const' command".into(),
+                        export_span,
+                    ));
+                    return (garbage_pipeline(spans), vec![]);
+                };
+
+                // Trying to warp the 'const' call into the 'export const' in a very clumsy way
+                if let Some(PipelineElement::Expression(
+                    _,
+                    Expression {
+                        expr: Expr::Call(ref def_call),
+                        ..
+                    },
+                )) = pipeline.elements.get(0)
+                {
+                    call = def_call.clone();
+
+                    call.head = span(&spans[0..=1]);
+                    call.decl_id = export_def_decl_id;
+                } else {
+                    working_set.error(ParseError::InternalError(
+                        "unexpected output from parsing a definition".into(),
+                        span(&spans[1..]),
+                    ));
+                };
+
+                let mut result = vec![];
+
+                if let Some(decl_name_span) = spans.get(2) {
+                    let decl_name = working_set.get_span_contents(*decl_name_span);
+                    let decl_name = trim_quotes(decl_name);
+
+                    if let Some(decl_id) = working_set.find_variable(decl_name) {
+                        result.push(Exportable::VarDecl {
+                            name: decl_name.to_vec(),
+                            id: decl_id,
+                        });
+                    } else {
+                        working_set.error(ParseError::InternalError(
+                            "failed to find added variable".into(),
+                            span(&spans[1..]),
+                        ));
+                    }
+                }
+
+                result
+            }
             _ => {
                 working_set.error(ParseError::Expected(
                     "def, def-env, alias, use, module, or extern keyword",
@@ -1589,6 +1642,9 @@ pub fn parse_module_block(
                                 None, // using commands named as the module locally is OK
                             ))
                         }
+                        b"const" => block
+                            .pipelines
+                            .push(parse_const(working_set, &command.parts)),
                         b"extern" | b"extern-wrapped" => {
                             block
                                 .pipelines
@@ -1713,6 +1769,9 @@ pub fn parse_module_block(
                                             module.add_submodule(name, id);
                                         }
                                     }
+                                    Exportable::VarDecl { name, id } => {
+                                        module.add_variable(name, id);
+                                    }
                                 }
                             }
 
@@ -1730,7 +1789,7 @@ pub fn parse_module_block(
                         }
                         _ => {
                             working_set.error(ParseError::ExpectedKeyword(
-                                "def, def-env, extern, alias, use, module, export or export-env keyword".into(),
+                                "def, const, def-env, extern, alias, use, module, export or export-env keyword".into(),
                                 command.parts[0],
                             ));
 
@@ -2208,7 +2267,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
         return (garbage_pipeline(spans), vec![]);
     };
 
-    let (import_pattern, module, module_id) = if let Some(module_id) = import_pattern.head.id {
+    let (mut import_pattern, module, module_id) = if let Some(module_id) = import_pattern.head.id {
         let module = working_set.get_module(module_id).clone();
         (
             ImportPattern {
@@ -2219,6 +2278,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                 },
                 members: import_pattern.members,
                 hidden: HashSet::new(),
+                module_name_var_id: None,
             },
             module,
             module_id,
@@ -2239,6 +2299,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                 },
                 members: import_pattern.members,
                 hidden: HashSet::new(),
+                module_name_var_id: None,
             },
             module,
             module_id,
@@ -2276,12 +2337,29 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                     id: *module_id,
                 }),
         )
+        .chain(
+            definitions
+                .variables
+                .iter()
+                .map(|(name, variable_id)| Exportable::VarDecl {
+                    name: name.clone(),
+                    id: *variable_id,
+                }),
+        )
         .collect();
 
     // Extend the current scope with the module's exportables
     working_set.use_decls(definitions.decls);
     working_set.use_modules(definitions.modules);
+    working_set.use_variables(definitions.variables);
 
+    let module_name_var_id = working_set.add_variable(
+        module.name(),
+        module.span.unwrap_or(Span::unknown()),
+        Type::Any,
+        false,
+    );
+    import_pattern.module_name_var_id = Some(module_name_var_id);
     // Create a new Use command call to pass the import pattern as parser info
     let import_pattern_expr = Expression {
         expr: Expr::ImportPattern(import_pattern),
@@ -2703,7 +2781,7 @@ pub fn parse_overlay_use(working_set: &mut StateWorkingSet, call: Box<Call>) ->
             )
         }
     } else {
-        (ResolvedImportPattern::new(vec![], vec![]), vec![])
+        (ResolvedImportPattern::new(vec![], vec![], vec![]), vec![])
     };
 
     if errors.is_empty() {

crates/nu-parser/src/parser.rs

@@ -2976,6 +2976,7 @@ pub fn parse_import_pattern(working_set: &mut StateWorkingSet, spans: &[Span]) -
         },
         members: vec![],
         hidden: HashSet::new(),
+        module_name_var_id: None,
     };
 
     if spans.len() > 1 {