Heavy cores are held incorrectly in first-person view

3D blocks are held at an angle when playing in first person model since there is a custom display transformation defined either in the model file or one of its referenced parents. However, heavy cores do not specify either of these, and are awkwardly held at the unusually straight default position. This is an obvious inconsistency with other blocks and should not be the case.

Contrast this with other blocks of similar sizes, such as dried ghasts, which are correctly held at an angle.

Interestingly, heads are affected by the same problem, which is tracked at MC-91869.

To fix, just introduce the relevant data for transformation in the heavy core model json file.