The `shift()` function removes the first element of the given source array.

Returns the removed value.

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

Throws a type exception if the given array is immuatable.

-- Testcase --
{%
	let arr = [ 1, null, 3 ];

	printf("%.J\n", [
		// remove one element
		shift(arr),

		// remove a null element
		shift(arr),

		// invalid source
		shift({ test: true })
	]);

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

-- Expect stdout --
[
	1,
	null,
	null
]
[
	3
]
-- End --