Switching to Github Actions

[Tzrlk]

Using github actions to execute branch and PR testing.

Currently:

• NodeJS 8 and 10 will require a mocha downgrade to function at all.
• 12 and below won't work with the newer stdlib functions.
• Everything else appears fine.

Had to mess around with the package locks, since the empty package name at the top causes a crash in versions below 16.

[Tzrlk]

Obviously this work depends on #425.

Also, I added the .editorconfig because my IDE was being a pain. Happy to remove it if desired.

[Tzrlk]

Put in a rudimentary shim in place to cater for nodejs 12. Low value, but low cost. Might be good to put babel in the build pipeline to add any needed polyfills instead of hand-rolling them.

[Tzrlk]

Alternately, the oldest version node.js actually maintains (found here) is 14, so dropping support for anything below that might be a solid option.

[Tzrlk]

I've removed the dependency on #425 so the node versions match those in the travis config, but I'd have to see about fixing the mocha execution issue mentioned before, since the exit code isn't propagated correctly at the moment. I'll add in one of the other changes I did to fix that, and see what I can do about downgrading mocha so it runs correctly.

[Tzrlk]

Ok, I give up. Fancy-pants browserslist scripting is just not meant to be.