* Add "orphaned" leaves nodes, these are copies of the regular leaves
nodes whose sole purpose is to replace leaves nodes when no more tree
trunks are present nearby. The orphaned nodes are swapped in by a
callback on destruction of tree trunk nodes and will be decayed by a
dedicated abm that processes all nodes in `group:leaves_orphan`.
* Add `mcl_core.update_leaves()` function, a `after_destruct` callback
handler for tree trunk nodes. The function finds leaves nodes nearby
destroyed tree trunk nodes and swaps these for orphaned leaves nodes
if no other tree trunk nodes remain near them.
* Add `after_destruct` callbacks to tree trunk node registrations in
mcl_core/nodes_trees.lua and mcl_mangrove/init.lua to update orphaned
leaves.
* Add entry for group `leaves_orphan` to GROUPS.md
* Also add entry for group `leaves` to GROUPS.md (it was missing).
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