Get the 1 based index of the first occurrence of the search string in the passed string. Returns -1 if the search string is not found.
The string to search
The string to match
The position in the string to start searching, -1 for the start of the string
The length of the section of string to search in, -1 for the whole string after StartPosition
xf.Str.IndexOf("The King's counting house is the King's", "King") => 5
xf.Str.IndexOf("The King's counting house", "den") => -1
xf.Str.IndexOf("The King's counting house", "King", 10, 8) => -1