Remove use of defaultValue

[javagl]

I'll open this despite the open question of What's the future of gltf-pipeline? (or rather, to prevent the short-term answer to that "being broken"):

The defaultValue function has been deprecated in CesiumJS, and is scheduled for removal in a few months. The gltf-pipeline library is stil used in several projects (with one of them being CesiumJS itself...). This PR removes the use of defaultValue throughout the codebase, and replaces it with the nullish coalescing operator.

(Note: This will likely cause merge conflicts with #669 . Depending on the timeline, we will have to fix the defaultValue deprecation first. In doubt, it should probably be possible to create a "replace-all"-regex for the changes here. I just did them manually, because they have just so been below the threshold where an automation seemed worthwhile, but that's subjective).

[lilleyse]

Thanks @javagl. We'll need to publish a new version of gltf-pipeline as well, right?

[javagl]

There are still the unknowns around #670

Given the interconnection between CesiumJS and gltf-pipeline, the concept of "versions" (or even "semantic versioning") don't have so much meaning here. Right now, users might declare
"cesium": "^1.103.0",
"gltf-pipeline": "^4.1.0",
(as, for example, in the 3d-tiles-tools). The gltf-pipeline itself declares
"cesium": "^1.86.1",
I think that this will cause trouble when Cesium 1.134 comes out (without defaultValue). And there will be further steps necessary to actually prevent this trouble. But I think that what has been done in this PR will be necessary, regardless of the exact further steps that will be taken.