Warn for interfaces that lack [Exposed=...] #27
Description
If interfaces lack [Exposed=...] they probably want to have [Exposed=Window] (which the warning could suggest) or something more elaborate if they need exposure in workers and such.
Starting to warn for this will allow us to remove [PrimaryGlobal] from IDL and get everyone to think this true as it's a pretty crucial part of API design.
Helps with whatwg/webidl#365.