chore: Extract PromptStore out of prompt_library (#25837)

One step closer to removing long pole with assistant/assistant2 builds

Release Notes:

- N/A

crates/assistant_slash_commands/src/default_command.rs

@@ -5,7 +5,7 @@ use assistant_slash_command::{
 };
 use gpui::{Task, WeakEntity};
 use language::{BufferSnapshot, LspAdapterDelegate};
-use prompt_library::PromptStore;
+use prompt_store::PromptStore;
 use std::{
     fmt::Write,
     sync::{atomic::AtomicBool, Arc},