Add types

[mikaelkaron]

This PR is an attempt to add TypeScript types for this package. I saw in a related ticket that it's preferred to add types directly to the package in question rather than @types so that's what I did.

JSDoc was taken from the README.md

[ChristianMurphy]

Thanks @mikaelkaron!
Could you include some type tests with dtslint?
There are some example of how this looks in some recent typing PRs across unified remarkjs/remark-external-links#18, remarkjs/remark-footnotes#4, remarkjs/remark-html#32

[mikaelkaron]

Stole some tests from test.js and updated the definitions to work with that.

[ChristianMurphy]

Excellent, thanks @mikaelkaron!
Let's get a test case in for rename.convert as well, and this will LGTM

[wooorm]

Released, thanks @mikaelkaron!

P.S. @ChristianMurphy I’ve been tagging things that add types as minor recently in the case of a package that’s likely not deep down in ancient versions of Node, and it’s going pretty well (as in, I haven‘t seen any issues)