Previously the range of MicrobitLightSensor read() was 0 - 100,
this didn't align with other APIs.
The new range is 0-255 which aligns with display brightness to give
a more consistent overall API.
* This commit fixes an exposed member variable
that was not intended to be public 'tickPeriod'
* normalised the light level to return values between 0 - 100
Added a new class called MicroBitLightSensor which interleaves sensing
with the display.
This class takes advantage of the previous commits which add a dynamic
system tick, and a new display mode for the display.
A future optimisation would be for the display to handle the instantion
of a light sensor when the display mode is changed!
