parser: factor out _trygetfunc() that extracts function name and arguments This provides a customization point for templater. In templater, there are two ways to call a unary function: func(x) and x|func. They are processed differently in templater due to historical reasons, but they should be handled in the same way while expanding aliases. In short, x|func should be processed as syntactic sugar for func(x). _funcnode and _getlist() are replaced by _trygetfunc().