Quarto: Add an API to set preferred editor (for respecting visual mode preference)

[jmcphers]

Currently, Quarto cannot set the visual editor as the default. A user can set it here, by right-clicking on a Quarto file .qmd file in the Explorer tab and choosing Open with...:

However, extensions can't change these. We want Quarto to have control over this behavior so that it can e.g. respect the editor YAML preference:

---
editor: 
  mode: visual
---

This will likely need to take the form of a Positron API.

[petetronic]

I believe this is related #1436

[mine-cetinkaya-rundel]

Just adding that preferred editor information can come from a single document or, in the case of a Quarto project, from the _quarto.yml file.

[isabelizimm]

I'm going to go ahead and close this issue, since we ended up NOT having to add any new code on the Positron side for supporting preferred editors in the quarto extension. See quarto-dev/quarto#577 for more info.