Introduce an annotation for overridability of GDScript functions

[Rob23oba]

Currently all GDScript functions can be overridden at will. I however propose that in the future, GDScript functions that should be overridable need to be marked with @virtual or a similar annotation, e.g. previously:

class MyClass:
	func some_animation(from: float, to: float, t: float) -> float:
		return lerpf(from, to, t)
class SubClass:
	extends MyClass

	func some_animation(from: float, to: float, t: float) -> float:
		return lerpf(from, to, t * t)

Now:

class MyClass:
	@virtual
	func some_animation(from: float, to: float, t: float) -> float:
		return lerpf(from, to, t)
class SubClass:
	extends MyClass

	func some_animation(from: float, to: float, t: float) -> float:
		return lerpf(from, to, t * t)

In this example, the class MyClass defined a simple linear interpolation function. However, it was overridden by SubClass to turn it into a quadratic interpolation. When looking at just the code of MyClass however, it is not obvious that this function is being overridden at all - one could think that the definition in MyClass is the one that is actually being used, however due to it being overridden this might not be the case. Introducing an @virtual annotation would solve this problem since it is now clearly documented that, indeed, that function can be overridden and one can now search such occurrences.
This change could also benefit performance, though: the GDScript compiler doesn't optimize very much currently but with this change it would be possible to inline short functions or at least replace function calls with direct references to the code.
Additionally, this makes GDScript functions similar to builtin-in functions that are also only overridable when they are defined as virtual.
However there is one major downside to this: all existing functions that can are being overridden would need to now be defined as @virtual. To simplify this, one could introduce some kind of "quick fix" tool that automatically adds @virtual where needed.

[AThousandShips]

Thank you for your proposal, consolidating in:

• Add GDScript support for defining virtual/prototype/must-override methods in base classes #1631

And a few other related proposals already open