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/examples/input.rs

@@ -635,7 +635,7 @@ impl Render for InputExample {
                     .flex()
                     .flex_row()
                     .justify_between()
-                    .child(format!("Keyboard {}", cx.keyboard_layout()))
+                    .child(format!("Keyboard {}", cx.keyboard_layout().name()))
                     .child(
                         div()
                             .border_1()