Add the `in` operator to check if a value is present in a mapping (as a
key) or sequence (as an element). This mirrors jq's `in` operator which
is defined as the inverse of `has`.
Usage:
. as $m | "key" | in($m) -- check map key existence
. as $m | "value" | in($m) -- check array value membership
.[] | select(.type | in(["a","b"])) -- filter by membership
Fixes#2322
