Returns a copy of the receiving string, with the characters from the nth character for length characters replaced with new. The replacement position and length can extend beyond the end of the receiving string. The starting position, n, is required and must be a positive whole number. The length argument is optional and must be a positive whole number or zero. If omitted, length defaults to the length of new.

If n is greater than the length of the receiving string, padding is added before the new string. The default pad character is a blank.

"abcdef"~replaceAt(" ",3, 1)      -->    "ab def"
"abcdef"~replaceAt(" ",3, 3)      -->    "ab f"
"abc"~replaceAt("123",5,6,"+")    -->    "abc+123"