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/inline_assistant.rs

@@ -29,6 +29,7 @@ use language_models::report_assistant_event;
 use multi_buffer::MultiBufferRow;
 use parking_lot::Mutex;
 use project::{CodeAction, ProjectTransaction};
+use prompt_library::PromptBuilder;
 use settings::{Settings, SettingsStore};
 use telemetry_events::{AssistantEvent, AssistantKind, AssistantPhase};
 use terminal_view::{terminal_panel::TerminalPanel, TerminalView};
@@ -42,9 +43,9 @@ use workspace::{notifications::NotificationId, ItemHandle, Toast, Workspace};
 use crate::buffer_codegen::{BufferCodegen, CodegenAlternative, CodegenEvent};
 use crate::context_store::ContextStore;
 use crate::inline_prompt_editor::{CodegenStatus, InlineAssistId, PromptEditor, PromptEditorEvent};
+use crate::terminal_inline_assistant::TerminalInlineAssistant;
 use crate::thread_store::ThreadStore;
 use crate::AssistantPanel;
-use crate::{prompts::PromptBuilder, terminal_inline_assistant::TerminalInlineAssistant};
 
 pub fn init(
     fs: Arc<dyn Fs>,