Use `anonymous_slot_expression` instead of `range_expression` for `extra_slots`

[sneakers-the-rat]

I had figured that range_expression would be suitable for expressing some subschema for allowable extra properties, but it turns out that range_expression -> anonymous_slot_expression does not actually have the range property. It seems like what we want is some anonymous_slot_expression, which works when i am monkeypatching the metamodel to implement it

[sneakers-the-rat]

i thought this was a valid use of slot_usage, but it may be the case that you can't alias a slot because the aliased slot then can't be found... by the new alias....

[amc-corey-cox]

@sneakers-the-rat I see that this is blocking another PR that seems pretty close to ready. This looks simple but there are CI failures. I'm not sure how to help out here. Can you give me an update on this PR? Thanks!