Returns the last index of "substr" inside of "string" counting backwards from the specified start index.
"string" - the string to search.
"substr" - the substring to search for.
start - the position from which to begin the search (default is the end of the string)
A scalar integer with the index. A failed search will return $null.
$last = lindexOf("abcdeab", "a"); println($last);
5