gpui: Add keybind metadata API (#33316)

Closes #ISSUE

Adds a very simple API to track metadata about keybindings in GPUI,
namely the source of the binding. The motivation for this is displaying
the source of keybindings in the [keymap
UI](https://github.com/zed-industries/zed/pull/32436).

The API is designed to be as simple and flexible as possible, storing
only a `Option<u32>` on the bindings themselves to keep the struct
small. It is intended to be used as an index or key into a table/map
created and managed by the consumer of the API to map from indices to
arbitrary meta-data. I.e. the consumer is responsible for both
generating these indices and giving them meaning.

The current usage in Zed is stateless, just a mapping between constants
and User, Default, Base, and Vim keymap sources, however, this can be
extended in the future to also track _which_ base keymap is being used.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/settings/src/settings.rs

@@ -15,7 +15,8 @@ pub use editable_setting_control::*;
 pub use json_schema::*;
 pub use key_equivalents::*;
 pub use keymap_file::{
-    KeyBindingValidator, KeyBindingValidatorRegistration, KeymapFile, KeymapFileLoadResult,
+    KeyBindingValidator, KeyBindingValidatorRegistration, KeybindSource, KeymapFile,
+    KeymapFileLoadResult,
 };
 pub use settings_file::*;
 pub use settings_store::{