Extract PromptLibrary to prompt_library (#23285)

This PR extracts the `PromptLibrary` out of the `assistant` crate and
moves it to the `prompt_library` crate.

The `PromptLibrary` is now decoupled from the specifics of the
`AssistantPanel` and `InlineAssistant`.

Release Notes:

- N/A

crates/prompt_library/Cargo.toml

@@ -16,6 +16,7 @@ anyhow.workspace = true
 assets.workspace = true
 chrono.workspace = true
 collections.workspace = true
+editor.workspace = true
 fs.workspace = true
 futures.workspace = true
 fuzzy.workspace = true
@@ -23,11 +24,20 @@ gpui.workspace = true
 handlebars.workspace = true
 heed.workspace = true
 language.workspace = true
+language_model.workspace = true
 log.workspace = true
+menu.workspace = true
 parking_lot.workspace = true
 paths.workspace = true
+picker.workspace = true
+release_channel.workspace = true
 rope.workspace = true
 serde.workspace = true
+settings.workspace = true
 text.workspace = true
+theme.workspace = true
+ui.workspace = true
 util.workspace = true
 uuid.workspace = true
+workspace.workspace = true
+zed_actions.workspace = true