Port to manifest v3

[Xwilarg]

Hey,

Following Chrome guideline, this extension will need to be ported to manifest V3 https://developer.chrome.com/docs/extensions/develop/migrate/what-is-mv3

I tried to see a bit how to do that and the main issue was that I am accessing background script this way:

(chrome.extension.getBackgroundPage() as any).downloadDoujinshi

And that getBackgroundPage was removed with manifest V3 https://stackoverflow.com/questions/68521873/chrome-extension-getbackgroundpage-doesnt-work-with-manifest-version-3

If someone is willing to help with these tasks it'ld be really appreciated :)
I made a branch called manifest-v3 around 10 months ago that should take care of some of the others points