Treeshake gsplat chunks

[LeXXik]

Currently gsplat shader chunks are included, even if its component system is not used. Would be great to treeshake those off.

[mvaligursky]

we need to create a collection for those chunks here (and also in wgsl folder)
https://github.com/playcanvas/engine/tree/main/src/scene/shader-lib/glsl/collections

and then handle imports to not include the collection outside of the gsplat shader generator or similar.