add highlighted color

Author: Thijss

src/power_grid_model_ds/_core/visualizer/layout/colors.py

@@ -9,4 +9,5 @@
     "selected_transformer": "#0349a3",
     "substation_node": "purple",
     "open_branch": "#c9c9c9",
+    "highlighted": "#a10000",
 }