feat(orderBy): add support for custom comparators

With this change the 3rd argument is interpreted as a comparator function, used to compare the
values returned by the predicates. Leaving it empty falls back to the default comparator and a
value of `false` is a special case that also falls back to the default comparator but reverses the
order. Thus the new implementation is backwards compatible.

This commit also xpands the documentation to cover the algorithm used to sort elements and adds
a few more unit and e2e tests (unrelated to the change).

Helps with angular#12572 (maybe this is as close as we want to get).

Fixes angular#13238
Fixes angular#14455

Closes angular#5123
Closes angular#8112
Closes angular#10368

Author: gkalpak