Chromium webext with manifest v3

[ffoodd]

Chrome will drop support for manifest v2 in next January, so we need to move to manifest v3. There're quite a few things to get right, and this probably means having separate code for Firefox—heavier than the current separate build only.

That's a fair point to get two distinct repositories: firefox-webext and chromium-webext, sharing only a few files probably. Getting a11y.css as a dependency would ease this quite a lot.

More info on manifest v3 and a dedicated guide for migrating to manifest v3,

BTW this might be the root cause of #18

[ffoodd]

Mozilla also shared some insights to migrate to manifest v3 on their blog and informed about manifest V3 support coming to Firefox Nightly in november.