USB-device-midi is missing helper function to send SysEx

Start with the USB-device-midi implementation and the linked pull request #1068, comparing the proposed helper with the supplied example. Verify that the helper handles the three SysEx packet endings and reports a full transmit buffer as shown; done means the helper is integrated without duplicating existing MIDI transfer behavior.

I was using USB-device-midi to send MTC to the host, which is essentially SysEx. But was slowed by the missing helper function to send the packet of SysEx. Anyhow, I wrote one...

Full example script here:
https://github.com/orgs/micropython/discussions/18450#discussioncomment-15043069

Happy to share with the project

    def send_sysex(self, p):
        # start of SysEx packet
        if len(p) > 3:
            w = self._tx.pend_write()
            if len(w) < 4:
                return False  # TX buffer is full. TODO: block here?

            w[0] = 0x4  # _CIN_SYSEX_START
            w[1] = p[0]
            w[2] = p[1]
            w[3] = p[2]
            self._tx.finish_write(4)
            self._tx_xfer()

            p = p[3:]

        # play out til end
        while p:
            if len(p) > 2:
                w = self._tx.pend_write()
                if len(w) < 4:
                    return False  # TX buffer is full. TODO: block here?

                w[0] = 0x7  # _CIN_SYSEX_END_3BYTE
                w[1] = p[0]
                w[2] = p[1]
                w[3] = p[2]
                self._tx.finish_write(4)
                self._tx_xfer()

                p = p[3:]
            elif len(p) > 1:
                w = self._tx.pend_write()
                if len(w) < 4:
                    return False  # TX buffer is full. TODO: block here?

                w[0] = 0x6  # _CIN_SYSEX_END_2BYTE
                w[1] = p[0]
                w[2] = p[1]
                w[3] = 0
                self._tx.finish_write(4)
                self._tx_xfer()

                p = p[2:]
            else:
                w = self._tx.pend_write()
                if len(w) < 4:
                    return False  # TX buffer is full. TODO: block here?

                w[0] = 0x5  # _CIN_SYSEX_END_1BYTE
                w[1] = p[0]
                w[2] = 0
                w[3] = 0
                self._tx.finish_write(4)
                self._tx_xfer()

                p = p[1:]

        return True