Allow trailing comma in function arguments

[olessavluk]

Currently it is valid to have trailing comma in function declaration, but not in function call. However according to spec it should work in both cases.

Example javascript code to reproduce this:

function a (
  a,
  b,
  c, // <- no error
) {
  console.log(params);
}

a(
  1,
  2,
  3,
); // <- error: "missing ) after argument list"

This feature was discussed before in #403 and later implemented in PR #410 .