feat: migrate to ES modules

[hlxid]

Closes codeoverflow-org/nodecg-io#829

This does run with node.js but only compiles using hacks and tests are failing because of (to me) mysterious errors.

Apparently, I am too dumb to understand ES modules given the time wasted on this, without getting it working.
The main two problems are:

1. types require the actual module import (e.g. axios but node.js doesn't seem to understand the default export and requires axios.default. See my FIXME comments in the code. For axios this might be fixed by upgrading to an axios version that is written is ES modules (haven't tested that yet) but this still remains with CodeBlockWriter.
2. Importing the isomorphic-git HTTP package fails in the tests because it assumes it to be commonjs and fails to parse the ES module of isomorphic-git even though es modules are enabled.

Config files (eslint, prettier, jest) are currently left at CommonJS and will be migrated if possible later.

I'll look into this later after some time has passed. Tips on what I am doing wrong are obviously very appreciated as well.