feat: add split_labelled_edge function

[leolaudouard]

In the same idea of the update_labelled_edge/5 function, I propose to add the split_labelled_edge/5 function

[leolaudouard]

I just updated the commit to have two separate functions under split_edge/4 & split_labelled_edge/5 because I noticed I introduced some regressions while trying to have only one function for both.

Happy to discuss/update the implementation if needed.