Manifest V3 support

[flytaly]

Until the end of the year, the extension should be ported to support the new Chrome manifest V3 API.

Currently, the extension uses webextension-polyfill under the hood to work in both Chrome and Firefox. For the time being, neither Firefox nor webextension-polyfill supports v3.

Relevant issues to track progress:

• Firefox,
• webextension-polyfill.

[flytaly]

#8