Raise deprecation for qr/(?foo})/

Raise deprecation for qr/(?foo})/

An unescaped left brace that is meant to be taken literally is
officially deprecated, though there are no plans to remove it in contexts
where we don't expect to use it to mean something else, and no warning
is raised in those contexts.

reg_mesg.t tests the known set of these contexts, currently (after this
commit):

 /^{/
 /foo|{/
 /foo|^{/
 /foo(:?{bar)/
 /\s*{/
 /a{3,4}{/

 This commit deprecates this context:

 /foo({bar})/

 This probably should have been illegal all along when 'bar' is a valid
 quantifier, as we do with the other quantifiers that follow a left
 paren whose illegality we haven't already taken advantage of to mean
 something else:

    qr/(+0)/
    Quantifier follows nothing in regex

This deprecation will allow ({...}) to be usable for a possible future
regex extension