Warn on dc:format

When `KHR_xmp` is present with the Dublin Core namespace, using `"dc:format"` is not recommended. Using this value means that GLB <-> .gltf converters will need to understand and modify this value when converting between `"model/gltf+json"` and `"model/gltf-binary"`. I think it would be better for exporters to omit the value, and for the Validator to issue a warning if it is present.

/cc https://github.com/KhronosGroup/glTF-Sample-Models/pull/263#discussion_r436937041

Start by locating the validator logic for KHR_xmp and the Dublin Core namespace, since no file or test is named in the issue. Check how an existing dc:format value is handled, then verify that its presence produces a warning and that the warning covers both relevant MIME values.

Written by the indexing model from the issue text.