Rename generated extension classes

[lilleyse]

Merge #372 first

Renamed all the generated extension classes based on discussion in #307 (comment)

The extension names in the existing code are pretty cumbersome. Maybe we should normalize them by prefixing with Extension and then a de-underscored, case-normalized version of the extension name. So EXT_meshopt_compression becomes ExtensionExtMeshoptCompression and 3DTiles_implicit_tiling becomes Extension3dtilesImplicitTiling. And when we have different types for extending different objects, ExtensionMeshPrimitiveExtFeatureMetadata. Still kinda cumbersome, but I don't have any better ideas short of totally manual names that wouldn't seem connected to the extension names at all, such as MeshPrimitiveFeatureMetadata for that last one.

I didn't implement the namespace idea because we would still have extensions starting with numbers that would need a prefix. And I wanted to keep the changes simple for now.

Daniel's idea of using namespaces is an interesting one! Put all the extensions in CesiumGltf::Extensions and then remove that Extension prefix?