Create reference implementation for proposed glTF extension BENTLEY_materials_line_style

[markschlosseratbentley]

Feature

Depends on CesiumGS/glTF#89.

Within CesiumJS, consider creating a reference implementation of the proposed glTF extension BENTLEY_materials_line_style as it is described at the above link.

This item should be completed after the above-linked PR is more fleshed out / fully proposed.