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/ui/src/components/icon.rs

@@ -192,6 +192,7 @@ pub enum IconName {
     WholeWord,
     XCircle,
     ZedAssistant,
+    ZedAssistantFilled,
     ZedXCopilot,
 }
 
@@ -315,6 +316,7 @@ impl IconName {
             IconName::WholeWord => "icons/word_search.svg",
             IconName::XCircle => "icons/error.svg",
             IconName::ZedAssistant => "icons/zed_assistant.svg",
+            IconName::ZedAssistantFilled => "icons/zed_assistant_filled.svg",
             IconName::ZedXCopilot => "icons/zed_x_copilot.svg",
             IconName::ArrowUpFromLine => "icons/arrow_up_from_line.svg",
         }