Assets: Walls (beginning)

wall_l_h (Wall Long Horizontal) and wall_s_h (Wall Short Horizontal)
have been added as prefabs under the enemy resources folder. However
when we start doing level design, we should be adding two additional
prefabs for vertical of long and short walls. Just needs rotations for
new pre-fabs. Centre of walls should be the xyz axis intersect so
rotation should be simple.