Support checking the absence of a key with =?

There are some instances where it's useful to assert on the shape of some keys of a map while also asserting that another key is not present.

https://github.com/search?q=repo%3Ametabase%2Fmetabase+%22key+is+not+present.%22&type=code

I'm not sure if it's simply a matter of `def` this or if there's magic missing
https://github.com/metabase/hawk/blob/main/src/mb/hawk/assert_exprs/approximately_equal.clj#L101C68-L101C108

Start at src/mb/hawk/assert_exprs/approximately_equal.clj around line 101, where the existing map assertion behavior is referenced. Trace how =? handles map keys and determine what is needed to express that a key is absent while checking other keys. Done means callers can use =? for this absence assertion alongside shape checks.

Written by the indexing model from the issue text.