Fix manifest v2 extension & release v2 artifact

[louisinger]

This PR fixes v2 version of the extension & adds a release artifact.

• dist & web-ext-artifacts folders are now divided into v2 & v3 directories, depending on which version of the extension you are building.
• add a web-ext:build:v3 script in pkg.json
• main.yml CI is now testing v2 AND v3 build & bundle.
• release.yml now upload two artifacts: marina-v3-{version}.zip & marina-v2-{version}.zip.

Unfortunately it's not possible to run the playwright tests under Firefox cause playwright supports extension context only on Chrome-based browsers.

please @tiero review this