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/conversions/fill.rs

@@ -143,7 +143,7 @@ fn fill(
     let cell_paths = (!cell_paths.is_empty()).then_some(cell_paths);
 
     let alignment = if let Some(arg) = alignment_arg {
-        match arg.to_lowercase().as_str() {
+        match arg.to_ascii_lowercase().as_str() {
             "l" | "left" => FillAlignment::Left,
             "r" | "right" => FillAlignment::Right,
             "c" | "center" | "m" | "middle" => FillAlignment::Middle,

crates/nu-command/src/conversions/into/bool.rs

@@ -116,13 +116,13 @@ fn into_bool(
 }
 
 fn string_to_boolean(s: &str, span: Span) -> Result<bool, ShellError> {
-    match s.trim().to_lowercase().as_str() {
+    match s.trim().to_ascii_lowercase().as_str() {
         "true" => Ok(true),
         "false" => Ok(false),
         o => {
             let val = o.parse::<f64>();
             match val {
-                Ok(f) => Ok(f.abs() >= f64::EPSILON),
+                Ok(f) => Ok(f != 0.0),
                 Err(_) => Err(ShellError::CantConvert {
                     to_type: "boolean".to_string(),
                     from_type: "string".to_string(),

crates/nu-command/src/conversions/into/datetime.rs

@@ -46,7 +46,7 @@ impl Zone {
         }
     }
     fn from_string(s: String) -> Self {
-        match s.to_lowercase().as_str() {
+        match s.to_ascii_lowercase().as_str() {
             "utc" | "u" => Self::Utc,
             "local" | "l" => Self::Local,
             _ => Self::Error,