Deep-merge two values. Plain objects merge recursively, arrays concatenate, and every
other value (Date, Map, Set, class instance, primitive) is replaced by the source
(cloned for plain structures, kept by reference otherwise). Pollution keys are skipped.
Deep-merge two values. Plain objects merge recursively, arrays concatenate, and every other value (Date, Map, Set, class instance, primitive) is replaced by the source (cloned for plain structures, kept by reference otherwise). Pollution keys are skipped.