Add qr/\b{gcb}/

Add qr/\b{gcb}/

A function implements seeing if the space between any two characters is
a grapheme cluster break.  Afer I wrote this, I realized that an array
lookup might be a better implementation, but the deadline for v5.22 was
too close to change it.  I did see that my gcc optimized it down to
an array lookup.

This makes the implementation of \X go from being complicated to
trivial.