Ignore macros with problematic argument specifications in listNewcommands

[theseanl]

For example, xparse allows specifying more than 1 characters as argument delimiters, such as in r{ab}c, but it simply ignores such macros. Trying to use a macro with such an argspec generates an error saying that there is no known macro with that name.

Currently, the argspec parser also accepts such argspecs, but for additional safety, we may implement some validator in unified-latex-utils-argspec which may be used in listNewcommands.

[siefkenj]

I think this is more an error in xparse. Has it been reported?