Implement our own pathfinder algorithm

[wofsauge]

The pathfinder is used to determine "unreachable" items.
The build-in pathfinder object of the game is only callable, if an EntityNPC is present. Thats why we spawn an invisible shopkeeper right now, which can cause issues in online coop & other mods.

Therefore, we should implement our own pathfinder, for example by using the A*-algorithm. Alternatively, we wait for Repentogon for Rep+, and use the Pathfinder provided by them.

I tested our a few git repos that implement A* in lua, but i couldnt get them to work reliable enough. therefore, this needs further investigation.