Extract PromptStore and PromptBuilder to new prompt_library crate (#23254)

This PR adds a new `prompt_library` crate and extracts the `PromptStore`
and `PromptBuilder` to it.

Eventually we'll want to house the `PromptLibrary` itself in this crate,
but right now that involves untangling a few dependencies.

Release Notes:

- N/A

crates/assistant/src/slash_command/prompt_command.rs

@@ -1,4 +1,3 @@
-use crate::prompt_library::PromptStore;
 use anyhow::{anyhow, Context, Result};
 use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
@@ -6,6 +5,7 @@ use assistant_slash_command::{
 };
 use gpui::{Task, WeakView};
 use language::{BufferSnapshot, LspAdapterDelegate};
+use prompt_library::PromptStore;
 use std::sync::{atomic::AtomicBool, Arc};
 use ui::prelude::*;
 use workspace::Workspace;