regexec.c: Revamp S_setup_EXACTISH_ST() loop end conditions

regexec.c: Revamp S_setup_EXACTISH_ST() loop end conditions

Consider the pattern /A*B/ where A and B are arbitrary.  The pattern
matching code tries to make a tight loop to match the span of A's.  The
logic of this was not really updated when UTF-8 was added.  I did
revamp it some releases ago to fix some bugs and to at least consider
UTF-8.

This commit changes it so that Unicode is now a first class citizen.
Some details are listed in the ticket GH #18414