Add support for KHR_mesh_quanitization about gltf

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Thanks for reporting. This model is using KHR_mesh_quantization which isn't handled by the reader utility.

The extension is marked as required in the example file, so the library ought report it as unsupported in some way. It begs the question: should the library reject any glTF that has unsupported extensions under extensionsRequired? In the next release there is an arbitrary extension API, so technically the burden would be passed onto the user.

A short term solution might be to fail creating the Reader for assets with extensions required matching *_mesh_*. Obviously, the long-term solution would be to add support for this extension.

[alteous]

#418 will now report an error if the crate can't handle a particular extension, including this one, so I've rebranded this issue as a feature request to add support for this feature.