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/assistant2/src/buffer_codegen.rs

@@ -1,10 +1,7 @@
 use crate::context::attach_context_to_message;
 use crate::context_store::ContextStore;
 use crate::inline_prompt_editor::CodegenStatus;
-use crate::{
-    prompts::PromptBuilder,
-    streaming_diff::{CharOperation, LineDiff, LineOperation, StreamingDiff},
-};
+use crate::streaming_diff::{CharOperation, LineDiff, LineOperation, StreamingDiff};
 use anyhow::{Context as _, Result};
 use client::telemetry::Telemetry;
 use collections::HashSet;
@@ -19,6 +16,7 @@ use language_model::{
 use language_models::report_assistant_event;
 use multi_buffer::MultiBufferRow;
 use parking_lot::Mutex;
+use prompt_library::PromptBuilder;
 use rope::Rope;
 use smol::future::FutureExt;
 use std::{