KHR_lights_punctual: the directional light needs a new property - angle

Now, many engines simulate realistic soft shadows that are cast by the directional light with an angle.

For examples, [UE4 - Directional Light - Light Source Angle](https://docs.unrealengine.com/en-US/Engine/Rendering/LightingAndShadows/LightTypes/Directional/index.html) and [Arnold - Directional Light - Angle](https://docs.arnoldrenderer.com/display/A5AFMUG/Directional+Light).

The issue proposes adding an angle property to KHR_lights_punctual directional lights and links Unreal Engine and Arnold references. No source file, schema location, or test is identified, so start by locating the KHR_lights_punctual definition in the glTF specification. Done would require an agreed property design and corresponding specification updates.

Written by the indexing model from the issue text.