implement autoloading (#13217)

# Description

This PR implements script or module autoloading. It does this by finding
the `$nu.vendor-autoload-dir`, lists the contents and sorts them by file
name. These files are evaluated in that order.

To see what's going on, you can use `--log-level warn`
```
❯ cargo r -- --log-level warn
    Finished dev [unoptimized + debuginfo] target(s) in 0.58s
     Running `target\debug\nu.exe --log-level warn`
2024-06-24 09:23:20.494 PM [WARN ] nu::config_files: set_config_path() cwd: "C:\\Users\\fdncred\\source\\repos\\nushell", default_config: config.nu, key: config-path, config_file_specified: None
2024-06-24 09:23:20.495 PM [WARN ] nu::config_files: set_config_path() cwd: "C:\\Users\\fdncred\\source\\repos\\nushell", default_config: env.nu, key: env-path, config_file_specified: None
2024-06-24 09:23:20.629 PM [WARN ] nu::config_files: setup_config() config_file_specified: None, env_file_specified: None, login: false
2024-06-24 09:23:20.660 PM [WARN ] nu::config_files: read_config_file() config_file_specified: None, is_env_config: true
Hello, from env.nu
2024-06-24 09:23:20.679 PM [WARN ] nu::config_files: read_config_file() config_file_specified: None, is_env_config: false
Hello, from config.nu
Hello, from defs.nu
Activating Microsoft Visual Studio environment.
2024-06-24 09:23:21.398 PM [WARN ] nu::config_files: read_vendor_autoload_files() src\config_files.rs:234:9
2024-06-24 09:23:21.399 PM [WARN ] nu::config_files: read_vendor_autoload_files: C:\ProgramData\nushell\vendor\autoload
2024-06-24 09:23:21.399 PM [WARN ] nu::config_files: AutoLoading: "C:\\ProgramData\\nushell\\vendor\\autoload\\01_get-weather.nu"
2024-06-24 09:23:21.675 PM [WARN ] nu::config_files: AutoLoading: "C:\\ProgramData\\nushell\\vendor\\autoload\\02_temp.nu"
2024-06-24 09:23:21.817 PM [WARN ] nu_cli::repl: Terminal doesn't support use_kitty_protocol config
```

# 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 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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> **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
<!-- If your PR had any user-facing changes, update [the
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-->

crates/nu-protocol/src/eval_const.rs

@@ -194,31 +194,11 @@ pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Valu
         // 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(
+        if let Some(path) = get_vendor_autoload_dir(engine_state) {
-            option_env!("NU_VENDOR_AUTOLOAD_DIR")
+            Value::string(path.to_string_lossy(), span)
-                .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
+            Value::error(ShellError::ConfigDirNotFound { span: Some(span) }, span)
-                        // 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", {
@@ -275,6 +255,41 @@ pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Valu
     Value::record(record, span)
 }
 
+pub fn get_vendor_autoload_dir(engine_state: &EngineState) -> Option<PathBuf> {
+    // 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
+    Some(
+        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),
+                    )
+                }
+            })
+            .into(),
+    )
+}
+
 fn eval_const_call(
     working_set: &StateWorkingSet,
     call: &Call,

src/config_files.rs

@@ -9,8 +9,9 @@ use nu_protocol::{
 };
 use nu_utils::{get_default_config, get_default_env};
 use std::{
+    fs,
     fs::File,
-    io::Write,
+    io::{Result, Write},
     panic::{catch_unwind, AssertUnwindSafe},
     path::Path,
     sync::Arc,
@@ -176,6 +177,46 @@ pub(crate) fn read_default_env_file(engine_state: &mut EngineState, stack: &mut
     }
 }
 
+fn read_and_sort_directory(path: &Path) -> Result<Vec<String>> {
+    let mut entries = Vec::new();
+
+    for entry in fs::read_dir(path)? {
+        let entry = entry?;
+        let file_name = entry.file_name();
+        let file_name_str = file_name.into_string().unwrap_or_default();
+        entries.push(file_name_str);
+    }
+
+    entries.sort();
+
+    Ok(entries)
+}
+
+pub(crate) fn read_vendor_autoload_files(engine_state: &mut EngineState, stack: &mut Stack) {
+    warn!(
+        "read_vendor_autoload_files() {}:{}:{}",
+        file!(),
+        line!(),
+        column!()
+    );
+
+    // read and source vendor_autoload_files file if exists
+    if let Some(autoload_dir) = nu_protocol::eval_const::get_vendor_autoload_dir(engine_state) {
+        warn!("read_vendor_autoload_files: {}", autoload_dir.display());
+
+        if autoload_dir.exists() {
+            let entries = read_and_sort_directory(&autoload_dir);
+            if let Ok(entries) = entries {
+                for entry in entries {
+                    let path = autoload_dir.join(entry);
+                    warn!("AutoLoading: {:?}", path);
+                    eval_config_contents(path, engine_state, stack);
+                }
+            }
+        }
+    }
+}
+
 fn eval_default_config(
     engine_state: &mut EngineState,
     stack: &mut Stack,
@@ -236,6 +277,8 @@ pub(crate) fn setup_config(
         if is_login_shell {
             read_loginshell_file(engine_state, stack);
         }
+        // read and auto load vendor autoload files
+        read_vendor_autoload_files(engine_state, stack);
     }));
     if result.is_err() {
         eprintln!(