Deprecate non-grapheme string delimiter

Deprecate non-grapheme string delimiter

In order for Perl to eventually allow string delimiters to be Unicode
grapheme clusters (which look like a single character, but may be
multiple ones), we have to stop allowing a single char delimiter that
isn't a grapheme by itself.  These are unlikely to exist in actual code,
as they would typically display as attached to the character in front of
them, but we should be sure.