Restrict white space inside [] inside(?[ ]) to \h

Restrict white space inside [] inside(?[ ]) to \h

This experimental construct has turned on /x processing for any interior
bracketed character classes, except comments are not allowed.  But,
bracketed character classes have traditionally all been on one line, and
I'm leery of the problems that could arise if we extend them to
multiple.  Therefore, restrict the white space to just spaces and tabs
before this feature becomes non-experimental.  If there is cause, we can
later relax the prohibition.