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-cmd-extra/src/extra/formats/to/html.rs

@@ -7,6 +7,7 @@ use nu_protocol::{
     record, Category, Config, DataSource, Example, IntoPipelineData, PipelineData,
     PipelineMetadata, ShellError, Signature, Spanned, SyntaxShape, Type, Value,
 };
+use nu_utils::IgnoreCaseExt;
 use rust_embed::RustEmbed;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
@@ -180,7 +181,7 @@ fn get_theme_from_asset_file(
     let th = asset
         .themes
         .into_iter()
-        .find(|n| n.name.to_lowercase() == theme_name.to_lowercase()) // case insensitive search
+        .find(|n| n.name.eq_ignore_case(theme_name)) // case insensitive search
         .unwrap_or_default();
 
     Ok(convert_html_theme_to_hash_map(is_dark, &th))