squares regex: Change domain specification to require zero match; length+=3
squares regex: Add new shortest regex in a separate file

This regex was devised by teukon. The main algorithm inside was developed by me, but the method of doing the test for divisibility by N at the end at the same time as the check that the main algorithm returns a result of zero, was developed by teukon.
squares regex: Rename short and fast version

It's not the shortest anymore, but it is the fastest one that is close in length to the shortest one.