modify the builtin `falling_node` entity to make it possible for mods to
register callbacks to be called when they land and turn into a node.
also register such a callback that checks if the landed node has an
entry `after_landing`, and if so calls that.
create `bg_api_fall` mod, and move files `falling_node.lua` and `loose_node.lua` to there from `bg_api`.
update dependencies in other mods to account for this change.
create public api function that registers "increasing ABM:s"; these
abstract over using numerical values associated with individual nodes in
ABM:s, allowing for straightforward creation of processes that occur
over time.
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.
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.
