Extended Block Shapes - Blockus

- Fit the new version Blockus 2.17.3 and 2.16.6, since Blockus has added stairs, slab and wall for some square blocks, this mod no longer adds these blocks. For these blocks, old block and item ids namespaced `extshape_blockus` are registered as an alias for ids namespace `blockus`. For example, block and item ID `extshape_blockus:marble_squares_slab` will be equivalent to `blockus:marble_squares_slab`.
    - Now blocks of chocolate blocks can be directly cut into blocks of chocolate squares in the stonecutter.
- See the update log for 3.1.6-beta.3.