The `rindex()` function locates an element within a given array or a substring
position within a given string, depending on the type of arguments given.

Returns `null` if the given haystack argument is neither an array nor a string,
returns `-1` if the element was not found within the array or the substring was
not found within the string.

Returns the last found index position in all other cases.

-- Testcase --
{%
	let o = {};

	printf("%.J\n", [
		rindex([ 1, 2, "abc", 3, "abc", 1, 2 ], "abc"),	// should return 4
		rindex([ 1, 2, 3 ], 4),							// should return -1
		rindex([ [], {} ], {}),							// should return -1 (strict equality)
		rindex([ [], o ], o),							// should return 1 (strict equality)

		rindex("foobarfoobarfoobar", "arf"),			// should return 10
		rindex("test", "hello"),						// should return -1
		rindex("test", "test"),							// should return 0 (needle = haystack length special case)
		rindex("test", ""),								// should return 4 (zero length needle special case)
		rindex("", ""),									// should return 0 (zero length special case)
		rindex("foo\0foo\0foo", "o\0f"),				// should return 6 (binary safe)

		rindex({ test: true }, true),					// should return null
		rindex(1234, 3),								// should return null
	]);
%}
-- End --

-- Expect stdout --
[
	4,
	-1,
	-1,
	1,
	10,
	-1,
	0,
	4,
	0,
	6,
	null,
	null
]
-- End --