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/prompt_library/src/prompt_library.rs

@@ -0,0 +1,11 @@
+mod prompt_store;
+mod prompts;
+
+use gpui::AppContext;
+
+pub use crate::prompt_store::*;
+pub use crate::prompts::*;
+
+pub fn init(cx: &mut AppContext) {
+    prompt_store::init(cx);
+}