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_context_editor/Cargo.toml

@@ -37,7 +37,7 @@ parking_lot.workspace = true
 paths.workspace = true
 picker.workspace = true
 project.workspace = true
-prompt_library.workspace = true
+prompt_store.workspace = true
 regex.workspace = true
 rope.workspace = true
 rpc.workspace = true