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Geometry Nodes: Add Mesh to Curve Node

This node creates poly curve splines from mesh edges. A selection
attribute input allows only using some of the edges from the mesh.
The node builds cyclic splines from branchless groups of edges where
possible, but when there is a three-way intersection, the spline stops.

The node also transfers all attributes from the mesh to the resulting
control points. In the future we could add a way to limit that to a
subset of the attributes to improve performance.

The algorithm is from Animation Nodes, written by @OmarSquircleArt.
I added the ability to use a selection, attribute transferring, and
used different variable names, etc, but other than that the algorithm
is the same.

Differential Revision: https://developer.blender.org/D11265