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Geometry Nodes: Add node labels to Attribute maths nodes

This adds the operator name to the node label which is consistent with the shading nodes.
The vector node has `Vector` as a prefix.

The Attribute nodes already have a different coloured header.

The same label is used when collapsing nodes, this helps readability.

Reviewed By: pablovazquez

Differential Revision: https://developer.blender.org/D10749