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/default_command.rs

@@ -0,0 +1,81 @@
+use super::{prompt_command::PromptPlaceholder, SlashCommand, SlashCommandOutput};
+use crate::prompt_library::PromptStore;
+use anyhow::{anyhow, Result};
+use assistant_slash_command::SlashCommandOutputSection;
+use gpui::{AppContext, Task, WeakView};
+use language::LspAdapterDelegate;
+use std::{
+    fmt::Write,
+    sync::{atomic::AtomicBool, Arc},
+};
+use ui::prelude::*;
+use workspace::Workspace;
+
+pub(crate) struct DefaultSlashCommand;
+
+impl SlashCommand for DefaultSlashCommand {
+    fn name(&self) -> String {
+        "default".into()
+    }
+
+    fn description(&self) -> String {
+        "insert default prompt".into()
+    }
+
+    fn menu_text(&self) -> String {
+        "Insert Default Prompt".into()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        &self,
+        _query: String,
+        _cancellation_flag: Arc<AtomicBool>,
+        _workspace: WeakView<Workspace>,
+        _cx: &mut AppContext,
+    ) -> Task<Result<Vec<String>>> {
+        Task::ready(Err(anyhow!("this command does not require argument")))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _argument: Option<&str>,
+        _workspace: WeakView<Workspace>,
+        _delegate: Arc<dyn LspAdapterDelegate>,
+        cx: &mut WindowContext,
+    ) -> Task<Result<SlashCommandOutput>> {
+        let store = PromptStore::global(cx);
+        cx.background_executor().spawn(async move {
+            let store = store.await?;
+            let prompts = store.default_prompt_metadata();
+
+            let mut text = String::new();
+            writeln!(text, "Default Prompt:").unwrap();
+            for prompt in prompts {
+                if let Some(title) = prompt.title {
+                    writeln!(text, "/prompt {}", title).unwrap();
+                }
+            }
+            text.pop();
+
+            Ok(SlashCommandOutput {
+                sections: vec![SlashCommandOutputSection {
+                    range: 0..text.len(),
+                    render_placeholder: Arc::new(move |id, unfold, _cx| {
+                        PromptPlaceholder {
+                            title: "Default".into(),
+                            id,
+                            unfold,
+                        }
+                        .into_any_element()
+                    }),
+                }],
+                text,
+                run_commands_in_text: true,
+            })
+        })
+    }
+}