Adding typehints and overloads for typing properties

[julianstirling]

Making ThingProperty itself a generic type seems to work for getting mypy to type a property, as long as you use typing @overload decorators to tell it when it is returning the descriptor rather than the value:

If we go with this, we need to think about how we remove model so we are not double defining they type.

This needs work to get MyPy passing on LabThings itself. The implementation is based off this stack exchange thread.

I minimised the number of inputs needed to Generic. But also ruff got angry with "Descriptor" as the type hint. So I tried using TypeVar("Descriptor") but this upsets MyPy. Need to understand exactly what should be done.

[julianstirling]

This was superseded by #127, so closing.

It is worth noting that #127 is now also closed as on review it was decided to give properties a larger underlying refactor to make their use more consistent and clear.