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 upgrade to Manifest V3 #93 (comment) 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.

[marcustyphoon]

Ugh.

To enable support for non-latest operating systems in Chrome, one needs to use different manifest files per-browser (as Filtering+ does); Chrome pre-121 will not load manifests with cross-platform background scripts (see https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background#browser_support). This would otherwise work in Chrome 102, which can be installed on Macs from 2008 and Windows 7 devices.