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/assistant/src/slash_command.rs

@@ -17,6 +17,7 @@ use std::{
 use workspace::Workspace;
 
 pub mod active_command;
+pub mod default_command;
 pub mod fetch_command;
 pub mod file_command;
 pub mod project_command;
@@ -117,6 +118,7 @@ impl SlashCommandCompletionProvider {
                                                 command_range.clone(),
                                                 &command_name,
                                                 None,
+                                                true,
                                                 workspace.clone(),
                                                 cx,
                                             );
@@ -178,6 +180,7 @@ impl SlashCommandCompletionProvider {
                                             command_range.clone(),
                                             &command_name,
                                             Some(&arg),
+                                            true,
                                             workspace.clone(),
                                             cx,
                                         );