gpui: Refactor PlatformKeyboardLayout (#29653)

Release Notes:

- N/A

crates/gpui/src/platform/keyboard.rs

@@ -0,0 +1,7 @@
+/// 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;
+}