The `index()` 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 first found index position in all other cases.

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

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

		index("foobarfoobarfoobar", "arf"),				// should return 4
		index("test", "hello"),							// should return -1
		index("test", "test"),							// should return 0 (needle = haystack length special case)
		index("test", ""),								// should return 0 (zero length needle special case)
		index("", ""),									// should return 0 (zero length special case)
		index("foo\0foo\0foo", "o\0f"),					// should return 2 (binary safe)

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

-- Expect stdout --
[
	2,
	-1,
	-1,
	1,
	4,
	-1,
	0,
	0,
	0,
	2,
	null,
	null
]
-- End --