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/assistant.rs

@@ -19,14 +19,13 @@ use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
 pub(crate) use model_selector::*;
-use prompt_library::PromptStore;
 pub(crate) use saved_conversation::*;
 use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
 use slash_command::{
-    active_command, fetch_command, file_command, project_command, prompt_command, rustdoc_command,
-    search_command, tabs_command,
+    active_command, default_command, fetch_command, file_command, project_command, prompt_command,
+    rustdoc_command, search_command, tabs_command,
 };
 use std::{
     fmt::{self, Display},
@@ -303,18 +302,10 @@ fn register_slash_commands(cx: &mut AppContext) {
     slash_command_registry.register_command(tabs_command::TabsSlashCommand, true);
     slash_command_registry.register_command(project_command::ProjectSlashCommand, true);
     slash_command_registry.register_command(search_command::SearchSlashCommand, true);
+    slash_command_registry.register_command(prompt_command::PromptSlashCommand, true);
+    slash_command_registry.register_command(default_command::DefaultSlashCommand, true);
     slash_command_registry.register_command(rustdoc_command::RustdocSlashCommand, false);
     slash_command_registry.register_command(fetch_command::FetchSlashCommand, false);
-
-    let store = PromptStore::global(cx);
-    cx.background_executor()
-        .spawn(async move {
-            let store = store.await?;
-            slash_command_registry
-                .register_command(prompt_command::PromptSlashCommand::new(store), true);
-            anyhow::Ok(())
-        })
-        .detach_and_log_err(cx);
 }
 
 #[cfg(test)]