ESP32 support for I2S

This previously existed as https://github.com/micropython/micropython-esp32/issues/185 but I'm replicating it here.  The ESP32 has I2S hardware so it'd be nice to support it!

References:
* [ESP-IDF docs](https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/peripherals/i2s.html)
* [MicroPython I2S API](https://github.com/micropython/micropython/wiki/Hardware-API#the-i2s-class)