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/vim/src/test/vim_test_context.rs

@@ -74,8 +74,12 @@ impl VimTestContext {
         .unwrap();
         cx.bind_keys(default_key_bindings);
         if enabled {
-            let vim_key_bindings =
-                settings::KeymapFile::load_asset("keymaps/vim.json", cx).unwrap();
+            let vim_key_bindings = settings::KeymapFile::load_asset(
+                "keymaps/vim.json",
+                Some(settings::KeybindSource::Vim),
+                cx,
+            )
+            .unwrap();
             cx.bind_keys(vim_key_bindings);
         }
     }