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/Cargo.toml

@@ -22,7 +22,6 @@ test-support = [
 
 [dependencies]
 anyhow.workspace = true
-assets.workspace = true
 assistant_settings.workspace = true
 assistant_slash_command.workspace = true
 assistant_tool.workspace = true
@@ -42,8 +41,6 @@ futures.workspace = true
 fuzzy.workspace = true
 globset.workspace = true
 gpui.workspace = true
-handlebars.workspace = true
-heed.workspace = true
 html_to_markdown.workspace = true
 http_client.workspace = true
 indexed_docs.workspace = true
@@ -63,6 +60,7 @@ parking_lot.workspace = true
 paths.workspace = true
 picker.workspace = true
 project.workspace = true
+prompt_library.workspace = true
 proto.workspace = true
 regex.workspace = true
 release_channel.workspace = true