feat(sterni/modules): module for fabric minecraft servers

This adds the module I've been using for running my minecraft servers.
It is inspired by the declarative minecraft server module in nixpkgs,
but

* does not support a non-declarative mode.

* supports more than one server on the same machine.

* patches the fabric mod loader into the server.jar on startup.

* its stopping mechanism is more robust: It issues a `save-all` and
  `stop` command over RCON and uses flock(1) for waiting on the
  server's shutdown instead of relying on checking for the PID
  via kill(1) in a loop.

It has some gaps in terms of features that I personally don't need, but
can be filled in over time.

Change-Id: I31b9139cab41a6398e5a08ecc72be33cd021ed2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7291
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/modules/default.nix

@@ -0,0 +1,2 @@
+# Stop readTree from looking at this directory
+_: { }