clean up things that were broken after renaming `bg_core` to
`bg_terrain`.
also clean up after global rename that added `bg_` prefix to every mod
name.
rename mods; add `bg_` prefix.
add hacky cleanup ABM (couldn't get LBM:s to work), that converts old
nodes into new ones according to this rename.
the cleanup doesn't do anything for items, though.
move plank registration and recipe out of tree mod into newly created
woodworking mod.
also update plank recipe to require cobblestone instead of stone to be
held while pummeling.
