Make SessionServer an environment property.

[CoreyShupe]

feather/feather/datapacks/src/vanilla.rs

Line 15 in cf650cc

"https://launcher.mojang.com/v1/objects/c5f6fb23c3876461d46ec380421e42b289789530/server.jar";

If possible there should be vanilla-mimic behavior here to allow for a session server to be defined via environment variables or CLI flags with the current as a default.