Allow NU_LIBS_DIR and friends to be const (#8310)

# Description

Allow NU_LIBS_DIR and friends to be const they can be updated within the
same parse pass. This will allow us to remove having multiple config
files eventually.

Small implementation detail: I've changed `call.parser_info` to a
hashmap with string keys, so the information can have names rather than
indices, and we don't have to worry too much about the order in which we
put things into it.

Closes https://github.com/nushell/nushell/issues/8422

# User-Facing Changes

In a single file, users can now do stuff like
```
const NU_LIBS_DIR = ['/some/path/here']
source script.nu
```
and the source statement will use the value of NU_LIBS_DIR declared the
line before.

Currently, if there is no `NU_LIBS_DIR` const, then we fallback to using
the value of the `NU_LIBS_DIR` env-var, so there are no breaking changes
(unless someone named a const NU_LIBS_DIR for some reason).


![2023-03-04-014103_hyprshot](https://user-images.githubusercontent.com/13265529/222885263-135cdd0d-7884-438b-b2ed-c3979fa44463.png)

# Tests + Formatting

~~TODO: write tests~~ Done

# After Submitting

~~TODO: update docs~~ Will do when we update default_env.nu/merge
default_env.nu into default_config.nu.

crates/nu-protocol/src/ast/call.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use serde::{Deserialize, Serialize};
 
 use super::Expression;
@@ -19,7 +21,7 @@ pub struct Call {
     pub redirect_stdout: bool,
     pub redirect_stderr: bool,
     /// this field is used by the parser to pass additional command-specific information
-    pub parser_info: Vec<Expression>,
+    pub parser_info: HashMap<String, Expression>,
 }
 
 impl Call {
@@ -30,7 +32,7 @@ impl Call {
             arguments: vec![],
             redirect_stdout: true,
             redirect_stderr: false,
-            parser_info: vec![],
+            parser_info: HashMap::new(),
         }
     }
 
@@ -70,10 +72,6 @@ impl Call {
         self.arguments.push(Argument::Positional(positional));
     }
 
-    pub fn add_parser_info(&mut self, info: Expression) {
-        self.parser_info.push(info);
-    }
-
     pub fn add_unknown(&mut self, unknown: Expression) {
         self.arguments.push(Argument::Unknown(unknown));
     }
@@ -106,8 +104,12 @@ impl Call {
         self.positional_iter().count()
     }
 
-    pub fn parser_info_nth(&self, i: usize) -> Option<&Expression> {
-        self.parser_info.get(i)
+    pub fn get_parser_info(&self, name: &str) -> Option<&Expression> {
+        self.parser_info.get(name)
+    }
+
+    pub fn set_parser_info(&mut self, name: String, val: Expression) -> Option<Expression> {
+        self.parser_info.insert(name, val)
     }
 
     pub fn has_flag(&self, flag_name: &str) -> bool {