Extras hint for mesh consolidation

In https://github.com/petrbroz/forge-convert-utils/issues/9 we're discussing the best way to provide hints for viewers and tools like https://github.com/zeux/meshoptimizer as to which scene nodes could be consolidated to improve runtime performance, and we'd like to get some feedback from the community.

We're thinking about adding a new field in the `extras`, something like `mergeGroup`. Another approach might be to provide some sort of a `consolidable` flag/hint on a node in the scene hierarchy that would suggest that it's the smallest "logical" entity, and that all its descendants can be consolidated. Has anyone else considered a similar type of hint?

Start by reading the linked forge-convert-utils issue and this discussion about hints for viewers and tools such as meshoptimizer. Compare the proposed extras.mergeGroup field with a node-level consolidable hint. There is no file or test named; done would require community and maintainer agreement on whether and how to specify the hint.

Written by the indexing model from the issue text.