feat: make Sand/Snow opaque and register Sand/Snow/Water blocks

World/Blocks/SandBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public class SandBlock : IBlock
+{
+    public ushort Id => 4;
+    public string Name => "Sand";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+    public TileCoord GetTextureTile(BlockFace face) => new(2, 1);
+}