Add pathfinding support

[Foxbud]

The HLD executable contains a lot of functions for manipulating paths and planning paths. These should be exposed as modding API.

This will likely involve giving mods the ability to register path indices. I'm not sure whether to force mods to register all of the path indices they might need during initializing or allow them to dynamically create and free paths. Furthermore, I'm not sure whether to present vanilla path indices as separate from mod paths or to just make them appear one-in-the-same.

This will also require allowing mods to attach "path end" event listeners to objects.