Cratification: Break out nu_cmd_lang into a separate crate (#8181)

# Description

This breaks out the core_commands into a separate crate called
nu_cmd_lang

_(Thank you for improving Nushell. Please, check our [contributing
guide](../CONTRIBUTING.md) and talk to the core team before making major
changes.)_

_(Description of your pull request goes here. **Provide examples and/or
screenshots** if your changes affect the user experience.)_

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-cmd-lang/src/core_commands/module.rs

@@ -0,0 +1,65 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
+};
+
+#[derive(Clone)]
+pub struct Module;
+
+impl Command for Module {
+    fn name(&self) -> &str {
+        "module"
+    }
+
+    fn usage(&self) -> &str {
+        "Define a custom module"
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("module")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .required("module_name", SyntaxShape::String, "module name")
+            .required("block", SyntaxShape::Block, "body of the module")
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        _call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::empty())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Define a custom command in a module and call it",
+                example: r#"module spam { export def foo [] { "foo" } }; use spam foo; foo"#,
+                result: Some(Value::test_string("foo")),
+            },
+            Example {
+                description: "Define an environment variable in a module",
+                example: r#"module foo { export-env { let-env FOO = "BAZ" } }; use foo; $env.FOO"#,
+                result: Some(Value::test_string("BAZ")),
+            },
+            Example {
+                description: "Define a custom command that participates in the environment in a module and call it",
+                example: r#"module foo { export def-env bar [] { let-env FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
+                result: Some(Value::test_string("BAZ")),
+            },
+        ]
+    }
+}