stack: stop disallowing '__V' and '__S' separators
stack: stop disallowing '__V' and '__S' separators

bin/modernish:
- Stop disallowing '__V' and '__S' as parts of a variable name
  for stack operations. This restriction is not actually necessary;
  since these strings are always added to variable names to generate
  the internal stack variable names, but never subsequently parsed
  using wildcards or otherwise, they are guaranteed to be unique
  in any case (provided you don't intentionally mess with the
  internal _Msh_* namespace, of course).

modernish/modernish