Make supersplat-viewer easier to use as a library

[d0rianb]

Hi, and thanks for this awesome work and support on the viewer.
I was wondering if there’s any plan (or interest) to evolve the repository into a more library-friendly module, where:

• key classes/utilities (like Viewer, Camera, Controller, etc.) are explicitly exported
• the public properties of the classes are exposed (ex: viewer.camera, viewer.activePose, ..)
• the project can be used as a proper ES module

If not, would you be open to contributions (via PR) that help refactor the codebase in that direction, without affecting existing usage?

[slimbuck]

If it can be done cleanly and we can still get what we need, sounds great!

Could you submit a PR with a flavour of the changes you're proposing so we can discuss concretely before you spend too much time on it?