keymap_ui: Add auto-complete for context in keybind editor (#34031)

Closes #ISSUE

Implements a very basic completion provider that is attached to the
context editor in the keybind editing modal.

The context identifiers used for completions are scraped from the
default, vim, and base keymaps on demand.

Release Notes:

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

crates/settings/src/keymap_file.rs

@@ -63,7 +63,7 @@ pub struct KeymapSection {
     /// current file extension are also supported - see [the
     /// documentation](https://zed.dev/docs/key-bindings#contexts) for more details.
     #[serde(default)]
-    context: String,
+    pub context: String,
     /// This option enables specifying keys based on their position on a QWERTY keyboard, by using
     /// position-equivalent mappings for some non-QWERTY keyboards. This is currently only supported
     /// on macOS. See the documentation for more details.