fix(type): Nullable setHighlightedIndex

[miya]

What:

Changed the argument (index) of setHighlightedIndex from number to number | null.

Why:

The implementation is supposed to allow null, but the interface did not accept null. Also it seems that a pull request implementing an approach that respects null, as also mentioned in #66 has been merged

How:

I've only made corrections to the interface.

Checklist:

• Documentation
• Tests
• TypeScript Types
• Flow Types
• Ready to be merged
  

[silviuaavram]

Why null? What's the use case to use null and not -1?

[miya]

Indeed, I think using -1 should be sufficient.
I've adjusted the interface to accommodate the fact that the implementation allows for null. If it's unnecessary, feel free to close.