TBD: Back to MV2?

[asamuzaK]

Related #282

• Sidebar Tabs is only for Firefox.
• https://bugzilla.mozilla.org/show_bug.cgi?id=1851083 is unlikely to be fixed soon.

There is no need to stick to MV3 and can go back to MV2?

[asamuzaK]

Ref #256

manifest.json:

• "action" -> "browser_action"
• "manifest_version": 3 -> 2

background.js

• action -> browserAction
• action.onClicked -> browserAction.onClicked

menu-items.js

• ['action'] -> ['browser_action']