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

Cargo.lock

@@ -339,7 +339,6 @@ dependencies = [
  "anthropic",
  "anyhow",
  "assistant_slash_command",
- "async-watch",
  "cargo_toml",
  "chrono",
  "client",
@@ -824,15 +823,6 @@ dependencies = [
  "tungstenite 0.16.0",
 ]
 
-[[package]]
-name = "async-watch"
-version = "0.3.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a078faf4e27c0c6cc0efb20e5da59dcccc04968ebf2801d8e0b2195124cdcdb2"
-dependencies = [
- "event-listener 2.5.3",
-]
-
 [[package]]
 name = "async_zip"
 version = "0.0.17"