Deprecate custom light component and use glTF punctual lights extension instead

[takahirox]

Is your feature request related to a problem? Please describe.

We use our custom light component but glTF has punctual lights extension. I don't think we really need the custon component for lights.

Describe the solution you'd like

Deprecate the custom light component, encourage to use glTF punctual lights extension instead, and let Spoke support the extension.

Benefits

• Less maintenance cost. We can reduce our own code for lighting and rely on imported Three.js and other modules supporting glTF punctual lights extension
• Better authoring model workflow. Models will have better compatibility and users will no longer need to learn/use Hubs specific authoring tools for lights.

Additional context

• The glTF lights extension doesn't support cast/receiveShadow property. We may still need custom component for them. (And we might be able to propose cast/receiveShadow extension to glTF community. I'm writing a draft.)
• glTF blender IO add-on doesn't seem to convert the lights power from blender to glTF correctly. We should work with them for better conversion. Intensity units when exporting point lights  KhronosGroup/glTF-Blender-IO#564 Maybe we want to deprecate the custom component after the blender exporter handles it correctly.