Add bag.member operator to theory of bags (#7857)

Add bag.member operator to theory of bags (#7857)

This PR adds the predicate bag.member to be analogous to predicate set.member.
The PR is motivated by converting regressions for sets to bags, which avoids defining a predicate for each set type

(define-fun bag.member ((e E) (B (Bag E))) Bool (>= (bag.count e B) 1))