expose an api function to register "loose" versions of existing nodes,
and implement some behaviour for such nodes; they will fall down if
there is air below them, and will eventually become their non-loose
counterparts if there is not air below them.
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.
every server step, mapblocks near online players will have randomly
selected nodes within them receive a "random tick", the behaviour of
which is determined in node definitions.
fix usage of `math.random` in various places to provide both a minimum
and a maximum number, instead of just maximum.
also renamed some files very slightly.
