Add support for `EXT_node_visibility_conditions`

I.e. this figure from the spec:


Multiple contents hasn't been widely adopted yet but there are a few examples where it might be used:

* OWT Well-Formed Format uses multiple contents to reference external tilesets
* Multiple contents + per-content timestamp metadata for time-dynamic 3D Tiles

No file, test, or entry point is named. Start by reading the EXT_node_visibility_conditions specification and the figure linked in the issue, then compare its requirements with the repository’s existing handling of multiple contents; done means the extension supports the described visibility conditions.

Written by the indexing model from the issue text.