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-color-config/src/nu_style.rs

@@ -578,7 +578,7 @@ fn fill_modifiers(attrs: &str, style: &mut Style) {
     //
     // since we can combine styles like bold-italic, iterate through the chars
     // and set the bools for later use in the nu_ansi_term::Style application
-    for ch in attrs.to_lowercase().chars() {
+    for ch in attrs.chars().map(|c| c.to_ascii_lowercase()) {
         match ch {
             'l' => style.is_blink = true,
             'b' => style.is_bold = true,