Improved tsconfig: Target the actual feature set of Node >=12

[voxpelli]

es6 also goes by the name of es2015 and eg. async/await didn't arrive until es2017 while Node 12 actually supports even es2019 fully.

Rather than keeping track of all of that in here, and ensuring to stay up to date here when new LTS versions gets released, I included a shared recommended minimal tsconfig as a base tsconfig for eg. that data.

On top of that I also added declarationMap to get source maps which maps from .d.ts to the original .ts files. This helps in debugging things and looking things up in eg. VSCode.

Sorry for not updating the lock file, my local yarn was acting up (I've moved to npm again myself since a few months so my new computer haven't really had yarn on it)

Also sorry for not using proper styling of the commit messages, maybe you can do a squash merge and/or cherry pick and set the correct commit messages then?

[AriPerkkio]

Thanks for the help @voxpelli. The tsconfig/bases looks really handy - I'm glad you brought that up. I'll definitely start to use those in my other projects as well.

I'll need to do some studying about the changes before merging. The CI is now happy so everything should be working fine. 👍