Support WebIDL partial namespaces

[fitzgen]

Blockers:

• Support WebIDL namespaces #253

https://heycam.github.io/webidl/#dfn-partial-namespace

namespace SomeNamespace {
  /* namespace_members... */
};

partial namespace SomeNamespace {
  /* namespace_members... */
};

Should be able to extend the AST we created for the original namespace in place.

[richard-uk1]

Implemented by #678

[alexcrichton]

👍

Thanks @derekdreery!