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/generators/generate.rs

@@ -150,9 +150,9 @@ used as the next argument to the closure, otherwise generation stops.
                             let mut err = None;
 
                             for (k, v) in iter {
-                                if k.to_lowercase() == "out" {
+                                if k.eq_ignore_ascii_case("out") {
                                     out = Some(v);
-                                } else if k.to_lowercase() == "next" {
+                                } else if k.eq_ignore_ascii_case("next") {
                                     next = Some(v);
                                 } else {
                                     let error = ShellError::GenericError(

crates/nu-command/src/generators/unfold.rs

@@ -162,9 +162,9 @@ used as the next argument to the closure, otherwise generation stops.
                             let mut err = None;
 
                             for (k, v) in iter {
-                                if k.to_lowercase() == "out" {
+                                if k.eq_ignore_ascii_case("out") {
                                     out = Some(v);
-                                } else if k.to_lowercase() == "next" {
+                                } else if k.eq_ignore_ascii_case("next") {
                                     next = Some(v);
                                 } else {
                                     let error = ShellError::GenericError(