Add: Preview attribute size and align

[MrBelarus]

Added Preview attribute size and align arguments. Size was in NaughtyAttributes and it was aligned differently.
Fixed exception if [Preview] attrubute used in generic class and object of generic class was [Button] attribute method as argument.
Updated samples, here is how it looks now:

using UnityEngine;
using Alchemy.Inspector;
using UnityEngine.UIElements;

namespace Alchemy.Samples
{
    public class PreviewSample : MonoBehaviour
    {
        [Preview(64, Align.FlexStart)] public Sprite foo;
        [Preview(64, Align.Center)] public Texture bar;
        [Preview] public Material baz;
        [Preview] public GameObject qux;
    }
}