vbscript: Support one-line sub/function

Sub / Function can be on one line, but only if there's brackets.

| Storage_opt tFUNCTION Identifier ArgumentsDecl BodyStatements tEND tFUNCTION

This leads to sometimes to link_statements being called with head being NULL, probably because now the : can be parsed as Statement in BodyStatements. Also note that this introduces another shift/reduce conflict, could be related, but I don't understand it well enough. Feedback would be appreciated, I'm still learning bison.