feature: Equivalent `x?//y` to `x? //y`

Because ?// only uses after as, and .name? //"unknown" isn't convenient enough

Possible Implements

Perhaps we can split the ?// token into two tokens, ? and //, if there is no ambiguity So, x as [$a] ? // {$a} can also be pass through

Alternatively, add a priority operation ?// and generate a ? // tree, operators with lower priority than ?// should behave consistently