New function radio.is_on()

Hi. While the display module has functions `on()` `off()` and [`is_on()`](https://microbit-micropython.readthedocs.io/en/latest/display.html#microbit.display.is_on), the radio module has only `on()` and `off()`. Have you considered adding an `is_on()` function to query the state?

To save power, my program turns the radio off in certain modes. I'd like to check whether the radio is on before calling receive(). Currently I keep track of that in a separate variable.