Add stricter nodeType type annotations

[tomdale]

This commit makes all of the node subclasses in simple-dom consistent with Element, which includes a more-refined nodeType property type annotation. By making these types enum values, it's easier for external systems to better type check passed values.

[krisselden]

I prefer this but it makes it so you can't pass a lib.dom.d.ts Element, Text, Document etc unless those have narrowed types Element { nodeType: 1 }