Extract SlashCommand trait from assistant (#12252)

This PR extracts the `SlashCommand` trait (along with the
`SlashCommandRegistry`) from the `assistant` crate.

This will allow us to register slash commands from extensions without
having to make `extension` depend on `assistant`.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -17,7 +17,6 @@ use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
-pub(crate) use prompts::prompt_library::*;
 pub(crate) use saved_conversation::*;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};