Support var in tuple destructuring

[JRI98]

To be consistent with other uses of var, I think it should also be supported in the context of tuple destructuring statements.

main! = |_| {
	Ok({})
}

g = |index| {
	(0, index)
}

f = |i| {
	(_, var $index) = g(i)
	(_, $index) = g($index)

	$index
}

➜  roc git:(main) ✗ roc check main.roc 
-- DUPLICATE DEFINITION --------------------------

The name $index is being redeclared in this scope.

The redeclaration is here:
   ┌─ .../roc/main.roc:11:6
   │
11 │    (_, $index) = g($index)
   │        ^^^^^^

But $index was already defined here:
   ┌─ .../roc/main.roc:10:6
   │
10 │    (_, var $index) = g(i)
   │        ^^^^^^^^^^

-- INVALID ASSIGNMENT TO ITSELF ------------------

The value $index is assigned to itself, which would cause an infinite loop at runtime.

Only functions can reference themselves (for recursion). For non-function values, the right-hand side must be fully computable without referring to the value being assigned.

   ┌─ .../roc/main.roc:11:18
   │
11 │    (_, $index) = g($index)
   │                    ^^^^^^


Found 1 error(s) and 1 warning(s) in 5693ms for main.roc.