Change the linux binary to xdg-open

[Abrifq]

Changelog

Changed the zeal option to xdg-open, which allows zeal on flatpak to pick up the requests. It also allows another dash-compatible app to run too.

Checklist

• [x] Code compiles correctly
• [X] Add or update tests, if possible
• [ ] All tests passing
• [X] Update the README, if necessary
• [X] New contributor? added myself via npm run contributor:add

[Abrifq]

How do I run the tests? :sweat_smile:

[farfromrefug]

@deerawan we really need this. Could you make a new release with this?