Add R3.CompositeDisposable support to AddTo extension

[YoruYomix]

Adds AddTo(R3.CompositeDisposable) overload for MotionHandle.

This allows motion handles to be added to R3's CompositeDisposable,
which automatically cancels the motion when disposed.

[nuskey8]

Wouldn't handle.ToDisposable().AddTo(compositeDisposable) be enough?

[YoruYomix]

It works in simple cases, but breaks down when you need to chain further after AddTo. A common real-world pattern is:

await LMotion.Create(0f, _originalColor.a, _duration)
  .WithEase(Ease.OutSine)
  .BindToColorA(_image)
  .AddTo(compositeDisposable)
  .ToUniTask(cancellationToken);

Without this overload, the chain must be split into separate statements because ToDisposable() loses the MotionHandle, making .ToUniTask() unreachable:

var handle = LMotion.Create(0f, _originalColor.a, _duration)
  .WithEase(Ease.OutSine)
  .BindToColorA(_image);

handle.ToDisposable().AddTo(compositeDisposable)

await handle.ToUniTask(cancellationToken);

Since AddTo(CompositeDisposable) returns this (MotionHandle), the chain stays intact — ToDisposable() breaks it by converting to IDisposable early. So while the workaround is functional, it forces boilerplate that the overload cleanly eliminates.