Add light property animations

[Alexeyt89]

Currently with "light" extension it is possible to save light nodes to glTF2.0
The nodes have "color" "intensity" "innerConeAngle" and "outerConeAngle" properties.
As EXT_property_animation extension has stuck in discussions for over a year and in the end it is not likelly to support light animations (#1301 (comment)) , I suggest that another extension should be created specifically to have the possibility to animate these properties.

[aaronfranke]

This is now possible thanks to the glTF Object Model and the KHR_animation_pointer extension.

For example, an animation can have the target set to this to animate the color of light 0:

"target": {
    "path": "pointer",
    "extensions": {
        "KHR_animation_pointer": {
            "pointer": "/extensions/KHR_lights_punctual/lights/0/color"
        }
    }
}

This issue can be closed as resolved.