Log a warning for interfaces and namespaces missing [Exposed]

[tobie]

In whatwg/webidl#423 we've started making [Exposed] mandatory (to favor explicit over implicit and avoid bugs where interfaces aren't exposed).

It would help move this forward if a warning was printed to the console when a namespace, non-callback interface, or callback interface with constants is missing its [Exposed] extended attribute.

As a second step, this warning could be turned into a fatal error.

[zcorpan]

Also see plinss/widlparser#27

[tabatkins]

This is done now, but you have to turn it on explicitly via Complain About: missing-exposed on. I need to do some refactoring of my metadata manager to enable it to be turned on by default, I think.