Manifest V3 draft

[marcustyphoon]

Worked through most of the pain points for cross-browser MV3. Took about two hours, apparently.

Notable notes, off the top of my head:

• webextension-polyfill is kind of a pain to load and it's equivalent to a oneliner now
• see comments in https://github.com/AprilSylph/Outbox-for-Tumblr/issues/93#issuecomment-2231999831 about optional_host_permissions key replacing optional_permissions key
• in Firefox, users can enable <all_urls> themselves in extension settings; in Chrome they can't. it is, as far as I can tell, literally impossible unless the extension calls browser.permissions.request. this is stupid.