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/slash_command/prompt_command.rs

@@ -1,5 +1,5 @@
 use super::{SlashCommand, SlashCommandCleanup, SlashCommandInvocation};
-use crate::prompts::prompt_library::PromptLibrary;
+use crate::prompts::PromptLibrary;
 use anyhow::{anyhow, Context, Result};
 use futures::channel::oneshot;
 use fuzzy::StringMatchCandidate;