Ship TypeScript definitions

[SnarkBoojum]

There are definitions available here, which have been discussed in #230 two years ago -- perhaps the contributor can update them and they could be shipped with the package?

Thanks!

[martincizek]

@domchristie Referring to this comment from you, do you think it is a good idea to have TS defs generated from JSDoc? We've managed to achieve it in the union-replacer project, where we struggled to introduce TypeScript tuples (it needs a postprocess script to generate it), but as long as Turndown only uses types described in pure JSDoc, it should be matter of reorganising JSDoc a bit....

[domchristie]

I suppose there's potential for us to get a bit tangled if we can't easily generate the correct definition, though I'd be interested to see how it goes, and revert to a hard-coded version if it becomes too much of a maintenance headache.