Create internal _is_utf8_mark()

Create internal _is_utf8_mark()

This is so we can deprecate non-core use of the existing one in a future
commit.  XS coders should be using the macros in handy.h instead of
calling such functions directly.  A future commit will deprecate all of
them, but first the core uses of this one must change so they don't
generate deprecation messages.  I will not have a chance to look for
some time, but I suspect that most uses of this function in the core
should be changed to use something else, but in the meantime, the
non-core uses can be deprecated.