chore: Simplify state machine implementation of prosemirror prompt input

[fkling]

In this PR I'm reducing the API surface of the state machine and move most of the events related to
value and mentions changes out of it. Hopefully this makes the state machine easier to understand
and to work with.

Now the state machine exposes an document.update event whose payload is a function that gets passed
the current editor state and returns a transaction. Note however that these changes actions are still
tested toghether with the state machine in promptInput.test.ts. I think it's important to have one
place to test all the expected behavior of the prompt input.

The main reason behind this change is that the current implementation is very much driven by the current
API the PromptInput component exposes. But going forward we might change/reduce/expand this API.
Ideally the state machine won't have to be changed for that.

In an upcoming PR I plan to refactor the mentions menu implementation to be more generic to better
support menu items that are not ContextItems.

Test plan

Unit test

[fkling]

This change is part of the following stack:

• chore: Simplify state machine implementation of prosemirror prompt input #7154 ◀

_{Change managed by git-spice.}

[thenamankumar]

LGTM