Update babylon-support branch to use es6 imports, module-level GTLF loader function, auto-detect plugin extension, and add/remove observables

[georginahalpern]

• Update dependency to es6 and more recent babylon version
• Update imports to avoid import * from BABYLON
• Update gltfLoader to use module-level gltfloader ImportFileAsync (to enable treeshaking)
• Update gltfLoader to detect plugin extension vs hardcode glb
• Integrate babylon observables into add/remove event listeners