Can Pointer.type be typed as a string union?

[zepumph]

It breaks modularity a bit, but it could be nice if event.pointer.type wasn't of type string, but instead `pdom'|'mouse'|'touch' etc. @jonathanolson could we have this type directly in Pointer?

The type checker is passing, so I thought I'd commit as a proposal.

I ran into this over in phetsims/molarity#245 where the pointer type was changed for PDOMPointer, but I missed a couple of cases. This probably won't happen again, but it may be nice to improve the typing here, so long as we don't expect users of scenery to add in their own Pointers ad hoc.

[jonathanolson]

This looks great, thanks!