This allows other mods to greater use the default.chest functions.
Fixes https://github.com/minetest/minetest_game/issues/1936
v2: Keep the default formspec name.
v3: Remove unnecessary backwards compatibility.
v4: register_lbm only applies to "default:chest" and
"default:chest_locked".
Move chests code out of nodes.lua and into a new file.
Credit torch code, and chest and torch models in README.txt.
Remove 'torches' mod text from torch.lua.
Minor cleanups in README.txt.