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/context/context_tests.rs

@@ -1,8 +1,8 @@
 use super::{AssistantEdit, MessageCacheMetadata};
 use crate::{
-    assistant_panel, prompt_library, slash_command::file_command, AssistantEditKind, CacheStatus,
-    Context, ContextEvent, ContextId, ContextOperation, InvokedSlashCommandId, MessageId,
-    MessageStatus, PromptBuilder,
+    assistant_panel, slash_command::file_command, AssistantEditKind, CacheStatus, Context,
+    ContextEvent, ContextId, ContextOperation, InvokedSlashCommandId, MessageId, MessageStatus,
+    PromptBuilder,
 };
 use anyhow::Result;
 use assistant_slash_command::{