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/Cargo.toml

@@ -16,7 +16,6 @@ doctest = false
 anyhow.workspace = true
 anthropic = { workspace = true, features = ["schemars"] }
 assistant_slash_command.workspace = true
-async-watch.workspace = true
 cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true