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",
         }

crates/ui/src/components/list/list_sub_header.rs

@@ -6,6 +6,7 @@ pub struct ListSubHeader {
     label: SharedString,
     start_slot: Option<IconName>,
     inset: bool,
+    selected: bool,
 }
 
 impl ListSubHeader {
@@ -14,6 +15,7 @@ impl ListSubHeader {
             label: label.into(),
             start_slot: None,
             inset: false,
+            selected: false,
         }
     }
 
@@ -28,12 +30,22 @@ impl ListSubHeader {
     }
 }
 
+impl Selectable for ListSubHeader {
+    fn selected(mut self, selected: bool) -> Self {
+        self.selected = selected;
+        self
+    }
+}
+
 impl RenderOnce for ListSubHeader {
-    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
+    fn render(self, cx: &mut WindowContext) -> impl IntoElement {
         h_flex().flex_1().w_full().relative().py_1().child(
             div()
                 .h_6()
                 .when(self.inset, |this| this.px_2())
+                .when(self.selected, |this| {
+                    this.bg(cx.theme().colors().ghost_element_selected)
+                })
                 .flex()
                 .flex_1()
                 .w_full()