TypeScript rewrite

[ExplodingCabbage]

Obviously this is barely started at the point of opening this PR. Criteria for merging:

• [ ] All src files are rewritten in TypeScript
• [ ] Restrict possible options properties correctly to each function
• [ ] The build step automatically compiles and still works as before
• [ ] Some kind of linting still runs
• [ ] Tests still run and pass
• [ ] Publishing to npm still works (obvs will test on a dummy package name)

I am not necessarily gonna get generated types for publishing to npm working in this PR; that can be a followup. I want to first of all complete and merge the smallest step in the right direction that doesn't make anything worse (which in this case will still be quite big).