Add ... hole token/expression

[thinker227]

Add ... as a "hole" which acts an an inline comment.

let x = ...;

func f(...) => ...;

func g(x) {
    print(x);
    ...
    print(y);
}

In place of an expression, ... could potentially evaluate to a runtime "not implemented" error. In any other place it should act as a comment without affecting surrounding code.