add function to make env vars case-insensitive (#14390)

# Description

This PR adds a new function that allows one to get an env var
case-insensitively. I did this so we can hopefully stop having problems
when Windows has HKLM as path and HKCU as Path.

Instead of just changing every function that used the original one, I
chose the ones that I thought were specific to getting the path. I
didn't want to go all in and make every env get case insensitive, but
maybe we should? 🤷🏻‍♂️

closes #12676

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crates/nu-cli/src/completions/command_completions.rs

@@ -41,8 +41,7 @@ impl CommandCompletion {
     ) -> HashMap<String, SemanticSuggestion> {
         let mut suggs = HashMap::new();
 
-        // os agnostic way to get the PATH env var
-        let paths = working_set.permanent_state.get_path_env_var();
+        let paths = working_set.permanent_state.get_env_var_insensitive("path");
 
         if let Some(paths) = paths {
             if let Ok(paths) = paths.as_list() {