WIP Playing around with .d.ts generation

[cspotcode]

Related to #4154

EDIT: got rid of the stuff about compiling; switched to emitDeclarationsOnly: true.

To play around with it locally, yarn nps build.types and yarn nps test.types

[coveralls]

Coverage increased (+0.07%) to 92.92% when pulling a03557364c15e02a52bfdfc186826bb49837a2b4 on cspotcode:dts into 579fd09db39a55b44c1f553df05c918bc62867be on mochajs:master.

[boneskull]

needs conflicts to be resolved

[voxpelli]

I like this idea, I think this is something to move forward with

[JoshuaKGoldberg]

👋 coming back to this @cspotcode, are you still interested in working on this PR? As of https://github.com/mochajs/mocha/issues/5027 there are again maintainers who can review it now. No worries if you no longer have time - but if you do that'd be great!