fix: associativity in bracketless kw list

[doorgan]

In the elixir parser there is a rule that allows when to bind to a bracketless keyword list(no_parens_op_expr -> when_op_eol call_args_no_parens_kw).

Spitfire doesn't have that rule, so this code:

+with a <- b, do: c when a (from property tests)

parses incorrectly as:

{
  :+,
  [line: 1, column: 1],
  [
    {
      :with,
      [line: 1, column: 2],
      [
        {
          :<-,
          [line: 1, column: 9],
          [{:a, [line: 1, column: 7], nil}, {:b, [line: 1, column: 12], nil}]
        },
        {:when, [line: 1, column: 21], [[do: {:c, [line: 1, column: 19], nil}], {:a, [line: 1, column: 26], nil}]}
      ]
    }
  ]
}

when Elixir produces this:

{
  :+,
  [line: 1, column: 1],
  [
    {
      :with,
      [line: 1, column: 2],
      [
        {
          :<-,
          [line: 1, column: 9],
          [{:a, [line: 1, column: 7], nil}, {:b, [line: 1, column: 12], nil}]
        },
        [do: {:when, [line: 1, column: 21], [{:c, [line: 1, column: 19], nil}, {:a, [line: 1, column: 26], nil}]}]
      ]
    }
  ]
}

The fix here is the same as #85, we need to use @list_comma to ensure the correct associativity for them.