Allow saving prompts from the Prompt Manager (#12359)

Adds the following features to the prompt manager:

- New prompt – Create a new prompt from the UI. It will only persist if
it is saved.
- Save prompt – Save a prompt by clicking the save button in the UI. A
keybinding will be added for this in the future.
- Reveal prompt - Show the selected prompt on the file system. Only
available for saved prompts.

New prompts that are saved will use the
`{slugified_title}_{ver}_{id}.md` format which all imported prompts will
move to in the near future.

Also orders prompts in alphabetical order by default.

Release Notes:

- N/A

crates/assistant/src/prompts.rs

@@ -1,3 +1,7 @@
-pub mod prompt;
-pub mod prompt_library;
-pub mod prompt_manager;
+mod prompt;
+mod prompt_library;
+mod prompt_manager;
+
+pub use prompt::*;
+pub use prompt_library::*;
+pub use prompt_manager::*;