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Geometry Nodes: Curve tangent node

This node outputs the direction vector, or tangent of a curve at every
control point. For poly splines this is simply the evaluated tangents,
so it all works very simply. For Bezier splines it uses the tangent at
the evaluated point corresponding to each control point, and NURBS
are interpereted as poly splines built from their control points.

Internally the node is called "Input Tangent" to simplify using it for
mesh tangents as well in the future like the "Normal" node.

Differential Revision: https://developer.blender.org/D12581