Need a way in the Object Model to read enum values

The Object Model provides access to some objects whose specific properties are determined by an enum value. KHR_lights_punctual light objects are a good example of this. The `spot/innerConeAngle` and `spot/outerConeAngle` properties are only available if the `type` is `spot`. For KHR_interactivity, we are missing a way to have logic that is conditional on these kinds of enum values because there are no rules in the object model for how to read the enum values.

I suggest one of two options:

1. Add the ability to read string data types to the object model and expose the enum values as read-only strings.
2. Expose the enums as integer values and provide a mapping from string to integer values in the object model.

Start with the Object Model rules and the KHR_lights_punctual and KHR_interactivity cases described in the issue. Compare the proposed string and integer enum-access approaches, then define the required object-model behavior and a way to verify conditional access to enum-dependent properties.

Written by the indexing model from the issue text.