Check dynamic const multi-assignment

[thomasmarshall]

This PR enables the dynamic assignment check for multi targets. The comments in the regression test said Sorbet doesn't do the dynamic constant workaround for multi-target assignments—that may have been the case when the original translator was added, but doesn't appear to be so now.

The following code was crashing:

def foo
  A, B = 1, 2
end

Enabling the dynamic assignment check for the multi target results in this being desugared in the same way as the original parser, and therefore no longer crashes:

Prism:

class <emptyTree><<C <root>>> < (::<todo sym>)
  def foo<<todo method>>(&<blk>)
    begin
      <assignTemp>$2 = [1, 2]
      <assignTemp>$3 = ::<Magic>.<expand-splat>(<assignTemp>$2, 2, 0)
      <dynamic-const-assign> = <assignTemp>$3.[](0)
      <dynamic-const-assign> = <assignTemp>$3.[](1)
      <assignTemp>$2
    end
  end
end

Original:

class <emptyTree><<C <root>>> < (::<todo sym>)
  def foo<<todo method>>(&<blk>)
    begin
      <assignTemp>$2 = [1, 2]
      <assignTemp>$3 = ::<Magic>.<expand-splat>(<assignTemp>$2, 2, 0)
      <dynamic-const-assign> = <assignTemp>$3.[](0)
      <dynamic-const-assign> = <assignTemp>$3.[](1)
      <assignTemp>$2
    end
  end
end

The error messages are also identical:

test.rb:2: dynamic constant assignment https://srb.help/2001
     2 |  A, B = 1, 2
          ^

test.rb:2: dynamic constant assignment https://srb.help/2001
     2 |  A, B = 1, 2
             ^
Errors: 2