Geometry Nodes: Fields version of attribute proximity node

Add a fields-aware implementation of the attribute proximity node.
The Source position is an implicit position field, but can be
connected with a position input node with alterations before use.
The target input and mode function the same as the original node.

Patch by Johnny Matthews with edits from Hans Goudey (@HooglyBoogly).

Differential Revision: https://developer.blender.org/D12635

release/scripts/startup/nodeitems_builtins.py

@@ -543,6 +543,7 @@ geometry_node_categories = [
         NodeItem("GeometryNodeLegacyDeleteGeometry", poll=geometry_nodes_legacy_poll),
         NodeItem("GeometryNodeLegacyRaycast", poll=geometry_nodes_legacy_poll),
 
+        NodeItem("GeometryNodeProximity"),
         NodeItem("GeometryNodeBoundBox"),
         NodeItem("GeometryNodeConvexHull"),
         NodeItem("GeometryNodeTransform"),