gpui: Introduce PlatformKeyboardLayout trait for human-friendly keyboard layout names (#29049)

This PR adds a new `PlatformKeyboardLayout` trait with two methods:
`id(&self) -> &str` and `name(&self) -> &str`. The `id()` method returns
a unique identifier for the keyboard layout, while `name()` provides a
human-readable name. This distinction is especially important on
Windows, where the `id` and `name` can be quite different. For example,
the French layout has an `id` of `0000040C`, which is not
human-readable, whereas the `name` would simply be `French`. Currently,
the existing `keyboard_layout()` method returns what's essentially the
same as `id()` in this new design.

This PR implements the `name()` method for both Windows and macOS. On
Linux, for now, `name()` still returns the same value as `id()`.

Release Notes:

- N/A

crates/settings/src/keymap_file.rs

@@ -195,7 +195,8 @@ impl KeymapFile {
     }
 
     pub fn load(content: &str, cx: &App) -> KeymapFileLoadResult {
-        let key_equivalents = crate::key_equivalents::get_key_equivalents(&cx.keyboard_layout());
+        let key_equivalents =
+            crate::key_equivalents::get_key_equivalents(cx.keyboard_layout().id());
 
         if content.is_empty() {
             return KeymapFileLoadResult::Success {