ManifoldCAD with wireframe and grid plane #5146

Thanks @diegoteran for keeping the releases going with 4.2! I recently upgraded ManifoldCAD.org to use the latest <model-viewer> and it's working great.

I also recall fielding requests occasionally when I was a maintainer here for things like a wireframe mode and other features that I deemed not core to the project. My response was usually something along the lines of "just hack down to our $scene symbol and you can do anything you want with Three.js". Well, I've decided to put my money where my mouth is and demonstrate it, since I have my own needs now as a user. You can follow the code here. And thanks to @swaparup36 for doing most of the implementation!