Refine style types

[kof]

Trying to make the types more accurate and ideally improve error messages and tsc performance in the end.

[kof]

This is a work in progress, before we can merge it I would like to turn all tests into ts files so that we have everything covered by the existing tests.

But I would love a pre-review already at this stage, because this is going to be a big PR.

[ITenthusiasm]

I haven't looked at the [currently last] commit 04e94fb0eace518e4c66e618d495acfa342e4a88 yet, but the preceding commits seem fine.

Regarding 04e94fb0eace518e4c66e618d495acfa342e4a88, it might be helpful to split out the prettier changes into a preceding (or succeeding) chore commit just to make the reviewing process a little easier. (A dev could speed through the prettier changes and then give more special attention to the type-related changes that are found in a separate commit.)