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/picker/src/picker.rs

@@ -236,7 +236,12 @@ impl<D: PickerDelegate> Picker<D> {
     /// If `scroll_to_index` is true, the new selected index will be scrolled into view.
     ///
     /// If some effect is bound to `selected_index_changed`, it will be executed.
-    fn set_selected_index(&mut self, ix: usize, scroll_to_index: bool, cx: &mut ViewContext<Self>) {
+    pub fn set_selected_index(
+        &mut self,
+        ix: usize,
+        scroll_to_index: bool,
+        cx: &mut ViewContext<Self>,
+    ) {
         let previous_index = self.delegate.selected_index();
         self.delegate.set_selected_index(ix, cx);
         let current_index = self.delegate.selected_index();