Is your feature request related to a problem? Please describe.
Sometimes when resource packs change things quite a bit, integrated PBR of some shaders (which works normally without explicit support, at least the ones in Complementary Shaders works well) won't perform very well, and resource-pack bundled PBR can help provide a correct rendering.

Describe the solution you'd like
Iris has already implemented the PBR support (Issue: IrisShaders/Iris#130, PR: IrisShaders/Iris#1401), there shouldn't be a big issue to port that to Forge (I assume, correct me if I'm wrong).