Export syntax for IDL extended attribute identifier values

[annevk]

In whatwg/html#4940 we are planning on adding values to [Serializable] and it would be nice if other standards could reference these values (and perhaps they need to be automatically linked through Bikeshed's IDL functionality).

[tabatkins]

Hm, kk. So this looks like it's actually a new dfn namespace, since the terms can overlap with anything else. And the values should be for="" the extended attribute.

Looks like I don't have a type for extended attributes themselves, either. I guess we want to be able to dfn those, and have IDL blocks autolink to them, too?

[annevk]

There's extended-attribute, right? Otherwise this sounds right to me.

cc @Ms2ger @domenic