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/strings/encode_decode/encoding.rs

@@ -28,7 +28,7 @@ pub fn decode(
     bytes: &[u8],
 ) -> Result<Value, ShellError> {
     // Workaround for a bug in the Encodings Specification.
-    let encoding = if encoding_name.item.to_lowercase() == "utf16" {
+    let encoding = if encoding_name.item.eq_ignore_ascii_case("utf16") {
         parse_encoding(encoding_name.span, "utf-16")
     } else {
         parse_encoding(encoding_name.span, &encoding_name.item)
@@ -45,7 +45,7 @@ pub fn encode(
     ignore_errors: bool,
 ) -> Result<Value, ShellError> {
     // Workaround for a bug in the Encodings Specification.
-    let encoding = if encoding_name.item.to_lowercase() == "utf16" {
+    let encoding = if encoding_name.item.eq_ignore_ascii_case("utf16") {
         parse_encoding(encoding_name.span, "utf-16")
     } else {
         parse_encoding(encoding_name.span, &encoding_name.item)
@@ -69,7 +69,7 @@ pub fn encode(
 
 fn parse_encoding(span: Span, label: &str) -> Result<&'static Encoding, ShellError> {
     // Workaround for a bug in the Encodings Specification.
-    let label = if label.to_lowercase() == "utf16" {
+    let label = if label.eq_ignore_ascii_case("utf16") {
         "utf-16"
     } else {
         label