add serialEventUSB to USART0_Handler

[morganrallen]

This will allow user code to define serialEventUSB making it more consistent with serialEvent for the other interfaces.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}