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/linux/headless/client.rs

@@ -9,7 +9,8 @@ use util::ResultExt;
 use crate::platform::linux::LinuxClient;
 use crate::platform::{LinuxCommon, PlatformWindow};
 use crate::{
-    AnyWindowHandle, CursorStyle, DisplayId, PlatformDisplay, ScreenCaptureSource, WindowParams,
+    AnyWindowHandle, CursorStyle, DisplayId, LinuxKeyboardLayout, PlatformDisplay,
+    PlatformKeyboardLayout, ScreenCaptureSource, WindowParams,
 };
 
 pub struct HeadlessClientState {
@@ -50,8 +51,8 @@ impl LinuxClient for HeadlessClient {
         f(&mut self.0.borrow_mut().common)
     }
 
-    fn keyboard_layout(&self) -> String {
-        "unknown".to_string()
+    fn keyboard_layout(&self) -> Box<dyn PlatformKeyboardLayout> {
+        Box::new(LinuxKeyboardLayout::new("unknown".to_string()))
     }
 
     fn displays(&self) -> Vec<Rc<dyn PlatformDisplay>> {