Consider merging with rollup-plugin-dts

[Swatinem]

Hi!

I recently created https://github.com/Swatinem/rollup-plugin-dts as an experiment, but it turned out quite well.
I would be willing to collaborate if possible :-)

It would actually solve #62 and maybe #54 and since it actually integrates with the type checker, doing something similar for the .js output would solve #28 and #109

Let me know what you think!