Add Time property and Preserve() method to MotionHandle

[AnnulusGames]

This PR adds a Time property to the MotionHandle, which allows you to manually control the playback of individual motions.

It also adds a Preserve() method to the MotionHandle, so that it won't be automatically destroyed after the motion is complete.

Example

public class Example : MonoBehaviour
{
    [SerializeField] Transform target;
    [SerializeField] Slider slider;

    MotionHandle handle;

    void Start()
    {
        handle = LMotion.Create(-5f, 5f, 1f)
            .WithEase(Ease.OutQuad)
            .BindToPositionX(target)
            .AddTo(target);

        handle.Preserve();
    }

    void Update()
    {
        handle.Time = slider.value;
    }
}