gh-36846: Resolve nice tree decomp bug in #36843, and allow `label_nice_tree_decomposition` to return a digraph
This PR aims to resolve #36843, i.e., it now handles potential join
nodes and singleton tree decomp correctly.
This PR also allows `label_nice_tree_decomposition` to return a directed
graph, and treats the root node as a `forget` node, simplifying
algorithm implementation.
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URL: https://github.com/sagemath/sage/pull/36846
Reported by: Jing Guo
Reviewer(s): David Coudert, Dima Pasechnik, Jing Guo
