I've written the base of air_quality API, next I will add a fake sensor for
testing the API, including unit testing for the fake sensor

I've removed the `AirQualityListener` and used a `Cell<Option<(u32,)>>`
instead. I've added a private Enum for choosing what kind of data reading
to execute, a private method that uses this Enum and two other public
functions as wrappers.

I've added a fake `AirQuality` driver and some integration tests for it.
Every test is passed and should reflect the behaviour of the real driver.

I've added unittests for each function of the AirQuality API, all pass
as expected. Documentation for the API will be part of the next commit.

Added the suggested changes, including the `read_sync` function that
return a tuple of both CO2 and TVOC values. Improved the unittests for the
fake driver and added a unittest for the new function.

I've added back the dedicated listener for this API
and rewritten the returning errors of some functions.

I've added some documentation for the functions of the API
and fixed an inclusion bug of the API in `tests.rs` that
made the CI fail.