Prompt library updates (#11988)

Restructure prompts & the prompt library.

- Prompts are now written in markdown
- The prompt manager has a picker and editable prompts
- Saving isn't wired up yet
- This also removes the "Insert active prompt" button as this concept doesn't exist anymore, and will be replaced with slash commands.

I didn't staff flag this, but if you do play around with it expect it to still be pretty rough.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <1789+nathansobo@users.noreply.github.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/assistant.rs

@@ -3,7 +3,6 @@ pub mod assistant_panel;
 pub mod assistant_settings;
 mod codegen;
 mod completion_provider;
-mod prompt_library;
 mod prompts;
 mod saved_conversation;
 mod search;
@@ -17,7 +16,7 @@ use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
-pub(crate) use prompt_library::*;
+pub(crate) use prompts::prompt_library::*;
 pub(crate) use saved_conversation::*;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};