GLTFLoader: Add KHR_mesh_primitive_restart

[donmccurdy]

For discussion of a proposed glTF extension, supporting primitive restart values in line loop, line strip, triangle strip, and triangle fan topologies:

• Add KHR_mesh_primitive_restart KhronosGroup/glTF#2569

Because three.js requires WebGL 2, no actual rendering changes are required, this PR just ensures that GLTFLoader recognizes the extension name. The KHR_mesh_primitive_restart proposal is meant to replace the existing EXT_mesh_primitive_restart extension, which would be more complex and less efficient to support in three.js.

Figure: A Hilbert space-filling curve, rendered in 3D. The asset is structured as a glTF scene and a single mesh primitive, containing a series of LINE_STRIP topologies separated by primitive restart values. Created with glTF Transform.

This contribution is funded by Bentley Systems.