Update no-unused-vars rule to allow ignored args

[morreman]

Suggestion: do not allow an argument before the last argument to be unused. This enforces all parameters to be used.
Allowing to ignore the unused variable with _

// Not allowed
function foo(a, b, c) { // 'b' is declared but its value is never read.
  console.log(a, c);
}

// Allowed
function foo(a, _b, c) {
  console.log(a, c);
}

[morreman]

Reference PR https://github.com/BonnierNews/jetsam/pull/438 that made me do this PR.