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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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
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- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used` to
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- `cargo test --workspace` to check that all tests pass (on Windows make
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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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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-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)