feat: support custom comparators

[fivecar]

Implements #2 in the current codebase.

Is it worth it?

Not sure. It increases the package size by about 80 bytes:

More importantly, perhaps, it makes every loop of shallowEqual* do a function call rather than use !==. If we decide we want this feature, but don't want the cost of the function call, we can instead introduce new shallowEqualCustom* functions that have the call, but leave the existing functions alone.