Geometry Nodes: Add shader Color Mix node

Port color mix shader node to Geometry Nodes.

Differential Revision: https://developer.blender.org/D10585

release/scripts/startup/nodeitems_builtins.py

@@ -495,6 +495,7 @@ geometry_node_categories = [
         NodeItem("GeometryNodeAttributeTransfer"),
     ]),
     GeometryNodeCategory("GEO_COLOR", "Color", items=[
+        NodeItem("ShaderNodeMixRGB"),
         NodeItem("ShaderNodeRGBCurve"),
         NodeItem("ShaderNodeValToRGB"),
         NodeItem("ShaderNodeSeparateRGB"),