Add all even subgrid sizes: 1/2, 1/4, 1/8, 1/16

[therappha]

Currently only subgrid_block (8×8×8) and subgrid_block_16 (16×16×16) are registered.

Add the remaining even sizes so all four variants are obtainable via /give:

Block	Grid	Scale
subgrid_block_2	2×2×2	1/2
subgrid_block_4	4×4×4	1/4
subgrid_block	8×8×8	1/8
subgrid_block_16	16×16×16	1/16

Also add corresponding tiny_stone_block_2 and tiny_stone_block_4 debug items.