Warn if a variable is not updated in the setter function

[heppocogne]

Related to godot-proposals #4940

• Add warning for when a member variable is not updated in the setter function.
• Because annotation does not support setter/getter functions, this warning currently cannot be suppressed by @warning_ignore().

[aaronfranke]

What happens with this valid use case?

var milliseconds = 0.0

var seconds:
	get:
		return milliseconds / 1000.0
	set(value):
		milliseconds = value * 1000.0

[heppocogne]

@aaronfranke
This PR gives a warning that seconds is not set in the setter function.

I think these codes should be warned in order to prevent mistakes, even if they are valid and works fine.
In addition, we can ignore warnings if they are unnecessary.

[jtnicholl]

Personally I disagree, if the variable is not referenced by either the setter or getter as in the example above then it isn't created. That should not generate a warning, it's not likely to be a mistake in that case. If the warning only appears when it's used in one or the other, I could maybe get behind this.
Also,

In addition, we can ignore warnings if they are unnecessary.

But you said yourself above:

• Because annotation does not support setter/getter functions, this warning currently cannot be suppressed by @warning_ignore().

IMO support for annotations on getters and setters should be added first, I don't like the idea of adding warnings that can't be suppressed.

[dalexeev]

• Related: Generate errors for getters of untyped variables returning nothing (superseded by #79363) #63541

[adamscott]

Post-poned the miletsone to 4.2 as this is an enhancement PR.

This PR needs a rebase too, as there's conflicts.