Add probe() function

[saschanaz]

This PR introduces probe() function which checks whether the next unconsumed token has the target type.

Currently checking excessive tokens requires using consume():

if (consume("?")) error("Can't nullable more than once");

Our error() method assumes that the error is from the first unconsumed token so it potentially throws with incorrect line number. The error message from the following example says it's from line 3 because the first unconsumed token is there.

interface X {
  attribute object?? // error is from line 2, but webidl2.js throws with line 3
    attributeName;
};

probe() will fix this by checking the next token without consuming it:

if (probe("?")) error("Can't nullable more than once");

[marcoscaceres]

nice!