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

Cerca nell’implementazione di String di ArduinoCore-API le dichiarazioni e le implementazioni di indexOf() e lastIndexOf(), quindi confronta i relativi tipi restituiti con len, charAt(), substring(), remove() e reserve(). Verifica che entrambi i metodi utilizzino in modo coerente il tipo unsigned previsto e che i chiamanti che passano i loro risultati a substring() non richiedano più un cast.

Scritto dal modello di indicizzazione a partire dal testo della issue.