Upgrade eslint, add prettier

[mroderick]

This PR replaces #262, as dependabot doesn't know how to upgrade when new dependencies are needed.

• Upgrade and add related plugins and configs
• Fix/disable lint violations

[fatso83]

@mroderick Merging this with all the prettier changes made a conflicting mess out of #237 (which no longer had any conflicts). Could we revert this, merge #237 and then re-merge (a rebased version of) this? I tried cleaning up by quickly resetting the branch, but it's protected (yeah, yeah, yeah), so I couldn't do this without anyone noticing ...

[mroderick]

@mroderick Merging this with all the prettier changes made a conflicting mess out of #237 (which no longer had any conflicts). Could we revert this, merge #237 and then re-merge (a rebased version of) this? I tried cleaning up by quickly resetting the branch, but it's protected (yeah, yeah, yeah), so I couldn't do this without anyone noticing ...

Sure, go ahead!

[fatso83]

Ha, seems @dominykas beat me to it, and actually fixed the conflicts.