export internal type definitions

[RikuVan]

This is a super library (thanks!) and, therefore, I am lightly wrapping it in a more idiomatic way for React (in a hook) and Svelte (using a store). The wrapper requires the same options as noUISlider so I have found myself reproducing the internal types where there are some small api tweaks It would be nicer to consume them all directly from noUISlider. I would be willing to make the PR if this seems reasonable to you. Alternatively, it is not a big deal to just pick them off the exported Options interface. So this not at all a blocking issue. But I think it would make it a little easier to extend some particular types.

[leongersen]

I'm very open to your suggestions on this. I've initially only exported the types I needed for the current set of examples and documentation, but I figure some other ones are externally useful. I'd welcome a PR!