Update extensions registry - #2642

@lexaknyazev Yes, maybe these?

• CesiumJS: https://github.com/CesiumGS/Cesium
• Cesium Native: https://github.com/CesiumGS/cesium-native
• @javagl's JSplat: https://github.com/javagl/JSplat
• @NorbertNopper-Huawei's 3dgs_ply2gltf tool: https://github.com/NorbertNopper-Huawei/3dgs_ply2gltf

Esri may have something to share for ArcGIS too, but I am unsure if any of their implementations are open source. Not sure what link they would want to use.

We don't need to list all known implementations, just something to replace the placeholder.

We had some discussions about this, and right now, I don't want to engage in arguments about what "what something being supported" means (and if so, that could/should happen at #2542). But people might want to try and load #2562 and expect the implementation to load them, and if this is not possible then it should be made clear. (I don't even know what kind of error message to expect there right now, but I'd place my bets on a DeveloperError being thrown)

@javagl The question for this PR is much simpler: would you prefer JSplat to be mentioned in the extension or not?

Prefer - compared to what?

Many extensions don't list "Known Implementations". Some just list "References", linking to papers, or GitHub permalinks to readers/writers (but from a quick sample, the latter is not used for the ratified ones). I'm still convinced that things like Draco don't even have two independent implementations.

In any case, this list could only be a "snapshot", as it will not be updated, and it could only be a formality, because it doesn't point to the relevant place in the code or make claims about whether the implementation is a reader, writer, or renderer.

So if a tool that can write the extension (but not read it) and a renderer that can render the SPZ extension (but not the base extension) count as "two implementations of the (base?) extension", in the spirit of the ratification conditions of the "Khronos Operational Guidelines", then that may be enough. JSplat can read, write, and render the base extension (but no other aspects of glTF, because it's focussed on splats). I frankly don't care whether or not it is included.

Bottom line: I have no strong preference here.

@weegeekps PTAL.