Make String::right count from pos instead of pos+1

- Change behavior of String:right to count from pos instead of pos+1
```
var s = "Godot"
print(s.right(2)) # return "dot" instead of "ot"
print(s.left(2), s.right(2)) # return "Godot" as usually expected, instead of "Goot"
```

- It might break existing script which reply on result from String::right