gltf-pipeline as an npm module?

@ebogo1 this is related to #9473

Today I updated `gltf-pipeline`, and noticed that the way we add the changes to Cesium are:

1. run `npm build-cesium` in `gltf-pipeline` (Not sure everything this build step does, but it does take some Node.js code involving `Buffer` and adapts it for the browser according to @lilleyse )
2. copy the output files into `Source/ThirdParty/GltfPipeline`
3. commit the changes

Could this workflow be changed to use `npm` instead? e.g. release `gltf-pipeline` and then `npm install` in `cesium`?

Start by tracing the `npm build-cesium` workflow in `gltf-pipeline` and how its output is consumed from `Source/ThirdParty/GltfPipeline` in Cesium. Compare that manual copy flow with installing a released npm package, and consider the work done when Cesium can build and use the dependency without committed copied output.

Written by the indexing model from the issue text.