Support JSON format

[aleokdev]

Support the Tiled JSON format as well, and add crate features for both XML and JSON loading, default to one of them, and mention it in docs/the README.

[bjorn]

Regarding this I think we can learn a lot from https://gitlab.com/flukejones/tiled-json-rs. I think it has a pretty good data structure setup. Unfortunately it also appears unmaintained since two years.

@flukejones Would you still be available to help?

I think it would be great if we had a single well-maintained Tiled crate with support for both TMX and JSON (ideally as features, so people can avoid overhead of what they don't need).

[flukejones]

It's not unmaintained per se, more like it's reached a point where I don't think there's anything to add (and it's still perfectly usable).

You're absolutely welcome to yeet any of the code I've written there in to this crate. If you do I'll add a little disclaimer directing people this way. Unfortunately I'm overworked so I'll maybe not be able to help.

I wouldn't worry too much about feature-gating, rust with LTO is very good at stripping out unused code.