Add links to reference implementations for all file formats

[Duckwhale]

Should probably link both the JS and LuaJIT versions, although I reckon the test cases/fixtures are more interesting. It might also be useful to link to third-party implementations (Go, Rust, Unity) etc., but it's difficult to ensure they actually work in all cases due to lack of tests. Even better, we could create a "shared" repository of decoders for all languages which would then share the test cases. This provides the highest benefit to the community, but it requires more collaboration (which is unlikely to happen overnight).

[Duckwhale]

Need to first finish the FFI ports. The old JS code is somewhat sketchy and I don't know which of the many versions is "complete".