99: add class element to tiledtile

[timjen3]

Added support for the class property to TiledTile. A TiledTile represents a tile in a tilemap. Long term it could make sense to take either of these alternative approaches:

• Check version number. If >= 1.9 then extract class. else extract type.
• Extract class if it exists, else extract type.

I took the approach of adding a redundant "class" property since this is consistent with how the library handles the class/type data in TiledMaps: https://github.com/TheBoneJarmer/TiledCS/blob/main/src/TiledMap.cs#L526

And since this is technically just an xml prop, it doesn't break anything to simply support both.

This closes #99