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/gpui/src/platform.rs

@@ -214,7 +214,7 @@ pub(crate) trait Platform: 'static {
     fn on_app_menu_action(&self, callback: Box<dyn FnMut(&dyn Action)>);
     fn on_will_open_app_menu(&self, callback: Box<dyn FnMut()>);
     fn on_validate_app_menu_command(&self, callback: Box<dyn FnMut(&dyn Action) -> bool>);
-    fn keyboard_layout(&self) -> String;
+    fn keyboard_layout(&self) -> Box<dyn PlatformKeyboardLayout>;
 
     fn compositor_name(&self) -> &'static str {
         ""
@@ -1634,3 +1634,11 @@ impl From<String> for ClipboardString {
         }
     }
 }
+
+/// A trait for platform-specific keyboard layouts
+pub trait PlatformKeyboardLayout {
+    /// Get the keyboard layout ID, which should be unique to the layout
+    fn id(&self) -> &str;
+    /// Get the keyboard layout display name
+    fn name(&self) -> &str;
+}