naming scheme for light properties in KHR_materials_common

[xelatihy]

In the schema for KHR_materials_common extension, light filenames names where defined as

light.XXX.schema.json

For consistency with the other schemas, I suggest to rename them with

light.XXX. KHR_materials_common.schema.json

This naming convention has the following benefits:

1. automatic code generation from the schema have the same names for properties in both lights and materials
2. should be backward compatible in validation

[pjcozzi]

Thanks @xelatihy, sorry for the slow response.

@lexaknyazev this change is OK with me but is it OBE given #632 (comment)?