Primitive interface

We actually don't have a concrete interface definition for `primitives`, i.e. things we store inside of `PrimitiveCollection`. It's simply documented everywhere as `Object`. What's even worse is that we have a `Primitive` class that renders geometry, but is not the base class or interface for all "primitives". Why we didn't name it `GeometryPrimitive` I'm not sure.

It would be great to clean up our own terminology here and define a base primitive interface so that we can use it in JSDoc and TypeScript.

No files, tests, or entry points are named. Start by tracing the PrimitiveCollection contents and the existing Primitive class, then review how primitives are represented in JSDoc and TypeScript. Done means the terminology is consistent and a shared primitive interface is defined for those usages.

Written by the indexing model from the issue text.