Support Chrome manifest v3

[cxw42]

• https://developer.chrome.com/docs/extensions/mv3/intro/mv3-migration/
• Timeline details: https://developer.chrome.com/docs/extensions/mv3/mv2-sunset/
• I note https://www.eff.org/deeplinks/2021/12/chrome-users-beware-manifest-v3-deceitful-and-threatening, but don't have a choice in the matter!

Depending on how manifest v3 shakes out, #100 might increase in priority.