Add Property Observers to GDScript

[Ericdowney]

When writing in a more reactive style using setget, I've found its cumbersome to have to write both the the get and the set.

I'm wondering if it would be possible to add will_set(new_value) and did_set(old_value) to GDScript properties to make reactive programming easier to understand. These property observers should work in addition to the existing get and set structure.

i.e.

# Implicit setget
var some_property: int : 
    will_set(new_value):
        # Implement
    did_set(old_value):
        # Implement

# Explicit setget with will / did sets
var other_property: Vector2 :
    get: return other_property
    set(new_value): other_property = new_value
    will_set(new_value):
        # Implement
    did_set(old_value):
        # Implement

This should also make things easier for beginners since it's easy to forget to set your property value in a set(new_value).