Add a `casePair` built-in function

[effectfully]

#5486 once merged will add caseList and caseData built-in functions. casePair is likely going to be less beneficial than either of caseList and caseData, because it'll only replace two builtin calls with a single one + a bunch of wiring, while the latter builtins replace much more than that, but it's likely that it'll still be worth it. So we should try adding casePair the same way as caseList and caseData, should be basically a simpler version of caseList.