Polish prompt library UX (#12647)

This could still use some improvement UI-wise but the user experience
should be a lot better.

- [x] Show in "Window" application menu
- [x] Load prompt as it's selected in the picker
- [x] Refocus picker on `esc`
- [x] When creating a new prompt, if a new prompt already exists and is
unedited, activate it instead
- [x] Add `/default` command
- [x] Evaluate /commands on prompt insertion
- [x] Autocomplete /commands (but don't evaluate) during prompt editing
- [x] Show token count using the settings model, right-aligned in the
editor
- [x] Picker 
- [x] Sorted alpha
- [x] 2 sublists
    - Default
        - Empty state: Star a prompt to add it to your default prompt
        - Otherwise show prompts with star on hover
    - All
        - Move prompts with star on hover

Release Notes:

- N/A

crates/gpui/src/platform/mac/window.rs

@@ -656,7 +656,7 @@ impl MacWindow {
                 .as_ref()
                 .and_then(|t| t.title.as_ref().map(AsRef::as_ref))
             {
-                native_window.setTitle_(NSString::alloc(nil).init_str(title));
+                window.set_title(title);
             }
 
             native_window.setMovable_(is_movable as BOOL);