String object, len is of type unsigned int, however indexOf() and lastIndexOf() return int

### Description

String Object:

len is of type unsigned int.
method that apply to position into the String or its size are expecting an unsigned int, this is the case with:
charAt()
substring()
remove()
reserve()

Oddly, indexOf() and lastIndexOf() are returning int and not unsigned int

### Current behavior

Oddly, indexOf() and lastIndexOf() are returning int and not unsigned int

### Expected behavior

indexOf() and lastIndexOf() ought to return unsigned int, to avoid unnecessary casting from int to unsigned int

### Additional information
It's frequent to pass the result of indexOf() to substring()

Thanks

Busca en la implementación de String de ArduinoCore-API las declaraciones y las implementaciones de indexOf() y lastIndexOf(), y compara después sus tipos de retorno con len, charAt(), substring(), remove() y reserve(). Verifica que ambos métodos usen de forma coherente el tipo unsigned previsto y que los llamadores que pasan sus resultados a substring() ya no necesiten un cast.

Escrito por el modelo de indexación a partir del texto del issue.