BREAKING CHANGE: merge *-types as idl-type

[saschanaz]

This should make implementing #570 easier since every IDL type object will have the same type string.

This patch closes #537 and includes:

• A relevant test
• A relevant documentation update

[marcoscaceres]

Approving, but it almost makes type: idl-type kinda meaningless... should we maybe just remove it? What's the expected fallout from this breaking change on WPT and ReSpec (I think we use it in ReSpec too)?

[saschanaz]

Closing, I guess something like instanceof IDLType should do the work.