Improve case insensitivity consistency (#10884)

# Description

Add an extension trait `IgnoreCaseExt` to nu_utils which adds some case
insensitivity helpers, and use them throughout nu to improve the
handling of case insensitivity. Proper case folding is done via unicase,
which is already a dependency via mime_guess from nu-command.

In actuality a lot of code still does `to_lowercase`, because unicase
only provides immediate comparison and doesn't expose a `to_folded_case`
yet. And since we do a lot of `contains`/`starts_with`/`ends_with`, it's
not sufficient to just have `eq_ignore_case`. But if we get access in
the future, this makes us ready to use it with a change in one place.

Plus, it's clearer what the purpose is at the call site to call
`to_folded_case` instead of `to_lowercase` if it's exclusively for the
purpose of case insensitive comparison, even if it just does
`to_lowercase` still.

# User-Facing Changes

- Some commands that were supposed to be case insensitive remained only
insensitive to ASCII case (a-z), and now are case insensitive w.r.t.
non-ASCII characters as well.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/src/help/help_.rs

@@ -10,6 +10,7 @@ use nu_protocol::{
     span, Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned,
     SyntaxShape, Type, Value,
 };
+use nu_utils::IgnoreCaseExt;
 #[derive(Clone)]
 pub struct Help;
 
@@ -144,7 +145,7 @@ pub fn highlight_search_in_table(
     highlight_style: &Style,
 ) -> Result<Vec<Value>, ShellError> {
     let orig_search_string = search_string;
-    let search_string = search_string.to_lowercase();
+    let search_string = search_string.to_folded_case();
     let mut matches = vec![];
 
     for record in table {
@@ -168,7 +169,7 @@ pub fn highlight_search_in_table(
                 }
                 let span = val.span();
                 if let Value::String { val: s, .. } = val {
-                    if s.to_lowercase().contains(&search_string) {
+                    if s.to_folded_case().contains(&search_string) {
                         *val = Value::string(
                             highlight_search_string(
                                 s,

crates/nu-command/src/help/help_commands.rs

@@ -134,7 +134,7 @@ fn build_help_commands(engine_state: &EngineState, span: Span) -> Vec<Value> {
         let usage = sig.usage;
         let search_terms = sig.search_terms;
 
-        let command_type = format!("{:?}", decl.command_type()).to_lowercase();
+        let command_type = format!("{:?}", decl.command_type()).to_ascii_lowercase();
 
         // Build table of parameters
         let param_table = {