Assorted fixes

[XhmikosR]

@jkphl: unsure if the package is still maintained but since we are still using it, I thought I'd make a PR. I could split some of the changes if needed.

There are failing tests even on master on Node.js >=12. I think the test PNGs need to be updated and also:

• svg2png; the current version breaks on Node.js 15 I updated it, but needs another pair of eyes
• node-sass; the current version does not work on Node.js 15
• mocha; still works though
• istanbul should be replaced with nyc because it's deprecated replace it with c8
• image-diff is deprecated and should be replaced too (could be one of the reasons image diffs fail on Node.js >= 12?)
• pn can be removed and use Node.js's util.promisify
• updated svgo to v2.x, needs another pair of eyes
• JSHint needs to be replaced at some point too

The rest of the deps shouldn't matter but my guess is Node.js < 10 support will need to be dropped. Edit, dropped it since more and more deps don't work there.

This should fix any lgtm.com issues. I also added a CodeQL action which is the successor to CodeQL, but ideally you could use both.

[XhmikosR]

@jkphl friendly ping :)

I can split some of the patches; I started this for a few fixes but I've landed quite a few patches, so it might be easier for you if I split the branch somehow. That being said, I tried to keep the patches as clean as possible.

Please also check the PR description.

[XhmikosR]

Closing this since I've split most patches. The only remaining patch is the svgo update which I'll tackle in the next days in a separate PR.