Port glTF exporter to cadquery

[fragmuffin]

cadquery models should be exportable to the gltf format.

Tasks

• Move cqparts.codec.gltf.ShapeBuffer over to cadquery.freecad_impl.exporters (somewhere)
• Build basic single layer scene exporter. Some of the cqparts hierarchical exporter code will be duplicated.

Note: this is a bit more of a cadquery issue... with respect to cqparts, this issue is considered closed when:

• ShapeBuffer is migrated to cadquery.
• cadquery PR is made, or an issue is raised.

[fragmuffin]

exporters are in the freecad_impl folder, so it seems to make sense to do this port as part of the OCC project.