[Finishes #75231370] Karel builder and stringified numbers

When editing karel levels, I might enter 01 in a grid.  Or, if I explicitly enter 0 it saves it as '0' but doesn't show it any differently than other 0 cells.  However, when we process it as a string instead of a number, we weren't always doing the right thing.  This would result in cases where we would allow the bee to fly off of the path.

It's not clear to me whether we need an additional step to convert existing levels.  I validated that if I edit a level and update (without any changes) it properly converts the stringified numbers to real numbers.  I'm not sure whether that code also gets run over the course of a deploy.  Do I need to create a migration?

Also, I changed the unit test to more clearly allow testing one value at a time.