JSONError should have structured way to add prefixes

[lilyball]

There are existing methods for doing parsing of objects and arrays that build up a full path on error. But if the user isn't using these particular methods, we lose that path. We should probably expose a way for the user to provide the path themselves. We'll probably want a more structured mechanism than the existing private withPrefix(_: String) method (e.g. perhaps we take an enum that has cases for object key vs array index).