Enable manifest v3

[rhelmer]

This is the branch we used to produce the 2.0 version pushed to the Chrome Web Store (CWS), let's merge it to main and use a branch for any future Firefox-only releases.

There are a few things we should resolve before landing it:

• [x] land the equivalent rally-sdk branch
• [x] land the equivalent web-science branch
• [x] fix the web-science rollup plugin so that it bundles content scripts correctly