- Backport implementation of the new model system from 1.15: (#6180)  - Extended block model format to support a "loader" value, as an alternative to "elements. - Extended block model format to support "gui_3d":false as a way to mark a model as being intended to be flat and lit from the front. (No this is not equivalent to the  "gui_light" flag in 1.15.2. It changes more than just the light direction) - Added a new OBJ loader designed from the ground up to work with the new model system. - Added stock models to serve as parent models when using custom loaders (forge:item/default, forge:item/default-tool, and forge:block/default). - Added stock white texture (forge:white) for use in loaders that want to draw untextured geometry. - Deprecated the old loader components, since they are gone in 1.15 already.