Limit Y level to prevent running off highway

As seen in [this](https://cdn.discordapp.com/attachments/732553144512413726/930203419598270474/unknown.png) image, baritone often runs off the side of a highway, and then loses tons of time walking below on the griefed floor. A solution would be to limit the y level that the bot could path to, so it won't go off the sides again.