multi-level mlhs treated as single level

This bug was found in opal/opal#1703

$ ruby-parse -e '((x, y)) = [[1,2]]'
(masgn
  (mlhs             # <- shouldn't this be (mlhs (mlhs ... ))?
    (lvasgn :x)
    (lvasgn :y))
  (array
    (array
      (int 1)
      (int 2))))
$ ruby-parse -e '(x, y) = [[1,2]]'
(masgn
  (mlhs            # <- the same as above, can't differentiate
    (lvasgn :x)
    (lvasgn :y))
  (array
    (array
      (int 1)
      (int 2))))
$ ruby-parse -V
ruby-parse based on parser version 3.0.2.0
$

This also happens in all legacy modes. This works correctly for ((x,y),) and for ((x,y),z)

Start by running the two ruby-parse reproductions shown in the issue and trace the parser entry point that builds the masgn/mlhs AST. The fix is done when ((x, y)) = [[1,2]] produces a nested mlhs distinct from (x, y) = [[1,2]], while the existing ((x,y),) and ((x,y),z) behavior remains correct in legacy modes.