The `unshift()` function places the given argument(s) at the begin of the
given array while maintaining their order.

Returns the last added value.

Returns `null` if the given destination argment is not an array.

Throws a type exception if the given array is immuatable.

-- Testcase --
{%
	let arr = [];

	printf("%.J\n", [
		// add one element
		unshift(arr, 123),

		// add multiple elements
		unshift(arr, 1, 2, 3),

		// add null values
		unshift(arr, null, null, 4),

		// no-op
		unshift(arr),

		// invalid destination
		unshift({}, 1, 2, 3)
	]);

	printf("%.J\n", arr);
%}
-- End --

-- Expect stdout --
[
	123,
	3,
	4,
	null,
	null
]
[
	null,
	null,
	4,
	1,
	2,
	3,
	123
]
-- End --