add a system level folder for future autoloading (#13180)

# Description

This PR adds a directory to the `$nu` constant that shows where the
system level autoload directory is located at. This folder is modifiable
at compile time with environment variables.
```rust
    // Create a system level directory for nushell scripts, modules, completions, etc
    // that can be changed by setting the NU_VENDOR_AUTOLOAD_DIR env var on any platform
    // before nushell is compiled OR if NU_VENDOR_AUTOLOAD_DIR is not set for non-windows 
    // systems, the PREFIX env var can be set before compile and used as PREFIX/nushell/vendor/autoload

    // pseudo code
    // if env var NU_VENDOR_AUTOLOAD_DIR is set, in any platform, use it
    // if not, if windows, use ALLUSERPROFILE\nushell\vendor\autoload
    // if not, if non-windows, if env var PREFIX is set, use PREFIX/share/nushell/vendor/autoload
    // if not, use the default /usr/share/nushell/vendor/autoload
```

### Windows default
```nushell
❯ $nu.vendor-autoload-dir
C:\ProgramData\nushell\vendor\autoload
```
### Non-Windows default
```nushell
❯ $nu.vendor-autoload-dir
/usr/local/share/nushell/vendor/autoload
```
### Non-Windows with PREFIX set
```nushell
❯ PREFIX=/usr/bob cargo r
❯ $nu.vendor-autoload-dir
/usr/bob/share/nushell/vendor/autoload
```
### Non-Windows with NU_VENDOR_AUTOLOAD_DIR set
```nushell
❯ NU_VENDOR_AUTOLOAD_DIR=/some/other/path/nushell/stuff cargo r
❯ $nu.vendor-autoload-dir
/some/other/path/nushell/stuff
```

> [!IMPORTANT] 
To be clear, this PR does not do the auto-loading, it just sets up the
folder to support that functionality that can be added in a later PR.
The PR also does not create the folder defined. It's just setting the
$nu constant.
 
# User-Facing Changes
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helps us keep track of breaking changes. -->

# Tests + Formatting
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Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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- `cargo run -- -c "use toolkit.nu; toolkit test stdlib"` to run the
tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
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crates/nu-cli/tests/completions/mod.rs

@@ -763,7 +763,7 @@ fn variables_completions() {
  // Test completions for $nu
  let suggestions = completer.complete("$nu.", 4);
 
- assert_eq!(17, suggestions.len());
+ assert_eq!(18, suggestions.len());
 
  let expected: Vec<String> = vec![
  "cache-dir".into(),
@@ -783,6 +783,7 @@ fn variables_completions() {
  "plugin-path".into(),
  "startup-time".into(),
  "temp-path".into(),
+ "vendor-autoload-dir".into(),
  ];
 
  // Match results

crates/nu-protocol/src/eval_const.rs

@@ -181,6 +181,46 @@ pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Valu
  },
  );
 
+ // Create a system level directory for nushell scripts, modules, completions, etc
+ // that can be changed by setting the NU_VENDOR_AUTOLOAD_DIR env var on any platform
+ // before nushell is compiled OR if NU_VENDOR_AUTOLOAD_DIR is not set for non-windows
+ // systems, the PREFIX env var can be set before compile and used as PREFIX/nushell/vendor/autoload
+ record.push(
+ "vendor-autoload-dir",
+ // pseudo code
+ // if env var NU_VENDOR_AUTOLOAD_DIR is set, in any platform, use it
+ // if not, if windows, use ALLUSERPROFILE\nushell\vendor\autoload
+ // if not, if non-windows, if env var PREFIX is set, use PREFIX/share/nushell/vendor/autoload
+ // if not, use the default /usr/share/nushell/vendor/autoload
+
+ // check to see if NU_VENDOR_AUTOLOAD_DIR env var is set, if not, use the default
+ Value::string(
+ option_env!("NU_VENDOR_AUTOLOAD_DIR")
+ .map(String::from)
+ .unwrap_or_else(|| {
+ if cfg!(windows) {
+ let all_user_profile = match engine_state.get_env_var("ALLUSERPROFILE") {
+ Some(v) => format!(
+ "{}\\nushell\\vendor\\autoload",
+ v.coerce_string().unwrap_or("C:\\ProgramData".into())
+ ),
+ None => "C:\\ProgramData\\nushell\\vendor\\autoload".into(),
+ };
+ all_user_profile
+ } else {
+ // In non-Windows environments, if NU_VENDOR_AUTOLOAD_DIR is not set
+ // check to see if PREFIX env var is set, and use it as PREFIX/nushell/vendor/autoload
+ // otherwise default to /usr/share/nushell/vendor/autoload
+ option_env!("PREFIX").map(String::from).map_or_else(
+ || "/usr/local/share/nushell/vendor/autoload".into(),
+ |prefix| format!("{}/share/nushell/vendor/autoload", prefix),
+ )
+ }
+ }),
+ span,
+ ),
+ );
+
  record.push("temp-path", {
  let canon_temp_path = canonicalize_path(engine_state, &std::env::temp_dir());
  Value::string(canon_temp_path.to_string_lossy(), span)