Localizable should be defined in WebIDL

[marcoscaceres]

If we do eventually get agreement on the shape of Localizable (#26), it really ought to be normatively specified in the WebIDL spec. The current document is a non-normative, and seems to suggest each spec should define their own Localizable, which is definitely not what we want.

The original Localizable was sent to WebIDL as a pull request a few years ago: whatwg/webidl#358

Should we make another attempt to revive that?

cc @ianbjacobs