Add 'as' prop to types

[klzns]

Add missing type for prop as.

This removes the need for the workaround proposed here:

• Using as prop throws error when using TypeScript #180

Also, it works better because the as prop is only available for styled components.

[ben-rogerson]

Thanks heaps for adding this PR - i'll include it within the next update.

[klzns]

@ben-rogerson not rushing or anything, but when do you plan to do the next update?

Thanks for your work! I'm a huge fan!

[ben-rogerson]

Next version is now published - thanks again for adding the missing type.
And great to hear you're enjoying twin 🎉