Add Model#toBuilder method

[Boy0000]

Convenience method to let you convert a Model to a builder for further manipulating it
Useful if you are trying to merge resourcepacks and avoid overriding one

So instead of this:

List<ItemOverride> overrides = new ArrayList<>();
Model baseModel = resourcePack.model(baseKey);
if (baseModel == null) Model.model().build();
Model.Builder modelBuilder = Model.model().key(baseModel.key()).parent(baseModel.parent()).textures(baseModel.textures());
modelBuilder.overrides(overrides);

You could do:

List<ItemOverride> overrides = new ArrayList<>();
Model baseModel = resourcePack.model(baseKey);
if (baseModel == null) Model.model().build();
baseModel = baseModel.toBuilder().overrides(overrides).build();