Make Element.type into an enum

[lukehutch]

Right now Element.type is a string. I keep running into new element types that I haven't seen before, and I have to add another else-if block to handle the additional element type. If this was an enum rather than a string, then the Dart tooling would show me which element types I still need to implement support for.

[lukehutch]

Maybe add the enum as another field, typeEnum or something, so that type is still backwards-compatible.