Forbid extension keywords from affecting the base IRI or creating embedded resources

[handrews]

While I am generally in favor of allowing extension keywords to do anything that JSON Schema Org keywords can do, I think we should clearly lock down resource identification and base IRI behavior. Messing with it would produce extremely unexpected behavior in an area that many implementations struggle to get right with just $id.

This issue does not address other possible ways that extension keywords might interact with IRIs (I'll be filing at least one more issue on this topic).