Update to manifest v3

[MrMamen]

Manifest v2 is deprecated and no longer accepted for releases:
https://developer.chrome.com/blog/mv2-transition/

This rewrite is required for #147

[rafaelgomesxyz]

This will be a bit of a problem because of how we use the background page at the moment, but I'll try to figure something out.

[rafaelgomesxyz]

Will try to tackle this. From an initial observation, this is what needs to be done:

• General changes to the manifest file. These will probably not be that difficult, as it's mostly renaming things and moving things around.
• Adapt any logic that required the background page to stay active (service workers go inactive after a while). Most of this is probably achievable by storing the data with browser.storage and then getting it when needed. Major refactor.
• Adapt script injections to use scripting.executeScript. Small refactor.
• Adapt any code that ran in intervals to use browser.alarms (like auto sync and cache checks running every hour). Small refactor.
• Adapt usages of webRequest to the new declarativeNetRequest. This one might be the more annoying one, but since webRequest is only used for Firefox, and only when using containers, I might just ignore it for now.