Also allow trailing comma after rest element

[hax]

Currently, let [a, ...b,] = iter or function f(a, ...b,) {} are syntax error.

As I understand, the intention of trailing comma is to allow move element easily when editing the code, especially in the code format like:

function f(
  a, 
  b,
  c,
) {}

it's easy to adjust the order of the whole line.

The rest element must be the last one, so there is no need to support trailing comma after rest element. It's actually protect programmers from appending elements after rest element.

But as this proposal allow a rest element be in any position, we should always allow trailing comma for consistency.

(Similar issue of other languages: python/cpython#53478 )