fix completions PATH vs Path (#8003)

# Description

This PR attempts to fix the completions issue where, on Windows,
completions wouldn't get generated from items in your path environment
variable. This seemed to be down to `PATH` vs `Path`. So, I tried to add
a new function that we can use anywhere to avoid this problem.

# 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-protocol/src/engine/engine_state.rs

@@ -449,6 +449,27 @@ impl EngineState {
         None
     }
 
+    // Get the path environment variable in a platform agnostic way
+    pub fn get_path_env_var(&self) -> Option<&Value> {
+        let env_path_name_windows: &str = "Path";
+        let env_path_name_nix: &str = "PATH";
+
+        for overlay_id in self.scope.active_overlays.iter().rev() {
+            let overlay_name = String::from_utf8_lossy(self.get_overlay_name(*overlay_id));
+            if let Some(env_vars) = self.env_vars.get(overlay_name.as_ref()) {
+                if let Some(val) = env_vars.get(env_path_name_nix) {
+                    return Some(val);
+                } else if let Some(val) = env_vars.get(env_path_name_windows) {
+                    return Some(val);
+                } else {
+                    return None;
+                }
+            }
+        }
+
+        None
+    }
+
     #[cfg(feature = "plugin")]
     pub fn update_plugin_file(&self) -> Result<(), ShellError> {
         use std::io::Write;