feat: add pathfinding exercise

[vaeng]

This is an idea I proposed on Discord as a reaction to @ErikSchierboom's call for exercises.

[BNAndras]

Degrees of separation make me think of https://en.m.wikipedia.org/wiki/Six_Degrees_of_Kevin_Bacon. Maybe we could add a Kevin as a homage?

[ErikSchierboom]

Lovely!

[jiegillet]

I've been trying to implement this one in Elixir, but I'm having trouble solving it, I think some cases may have issues.

1. "Direct parent-child relation" => degree 1, no problem
2. "Sibling relationship" => expected degree between siblings is 1, but from the description I expected it to be 2, with the path being: left child => parent => right child. Bypassing the parent for siblings is fine as a rule, but it should be explicit.
3. "Two degrees of separation, grandchild" => degree 2, no problem
4. "Unrelated individuals" => no problem
5. "Complex graph, cousins" => degree 4, but my solution gave degree 9. I did it by hand and I also get 9 (with the rule of siblings having a degree of separation 1, otherwise you would get 10)
6. "Complex graph, no shortcut, far removed nephew" => degree 15, but my solution fives me 14 (it could be 15 without the siblings rule potentially, I did not do this one by hand)
7. "Complex graph, some shortcuts, cross-down and cross-up, cousins three times removed" => degree 8, but my solution gives me no connection, and I confirmed by hand. Sofia doesn't have a parent, and she starts a second family with Qi in it.

@vaeng could you help me here?

Also in general, I like having large inputs, but only at the end when all base case have been covered. The cousin case, for example could be much smaller, it took me a long time to verify by hand.
I also noticed that the Elixir exercise generator does not preserve the ordering of the family tree entries, but with random order I never would have been able to check my solutions by hand, so maybe it's worth adding a comment saying that preserving the order would be a good idea.

Here is my tentative implementation.