Move node-tmx-parser to optionalDependencies and unify with Tiled's Native JSON support

[toastal]

I was looking at doing this, but node-tmx-parser is similar but not the same API as the what's exported from Tiled. I'm pretty new to Tiled so I don't know the nuances I'd be missing. The JSON format should likely be preferred though since it doesn't involve expensive XML parsing on the client.