Manifest v3

[teidesu]

the previous pr #1608 for some reason stalled and was never merged.
i too am interested in seeing sidebery working under chrome :3

i picked up after them and rebased, though it seems as if most of the issues you raised in that pr have already been addressed
i only really updated the bg script to set up listeners in top-level to (hopefully) work correctly with mv3 event pages

a few open questions i still have:

• how to test that non-persistent background script works fine? i.e. what features rely on the background script being persistent?
• "Record state changes in local storage" – i think it's already storing everything it needs in local storage? not sure
• "Change timers to alarms" – seems like there aren't any long-running timers in the background script?
• "Place menu creation [...] in a runtime.onInstalled listener" – haven't really changed anything there, but menus seem to be working fine

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if we're going the "chrome support" route, we'll eventually need to get rid of webRequest/webRequestBlocking api usages too.
i don't entirely understand where they are currently used, seems like something related to containers..? if so, can probably be skipped for now as chrome doesn't have anything like that

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will close #916