[perl #94490] const fold should not trigger special split " "

[perl #94490] const fold should not trigger special split " "

The easiest way to fix this was to move the special handling out of
the regexp engine.  Now a flag is set on the split op itself for
this case.  A real regexp is still created, as that is the most
convenient way to propagate locale settings, and it prevents the
need to rework pp_split to handle a null regexp.

This also means that custom regexp plugins no longer need to handle
split specially (which they all do currently).