Allow descent below Y=0

[SlimeDog]

Describe the bug
Void starts at Y=-59 in MC 1.18. VoidSpawn traps at Y=0.

To Reproduce
Steps to reproduce the behavior:

1. Upgrade to Spigot 1.18
2. Change gamemode to spectator (just to make it easier to descend)
3. Attempt to descend below Y=0
4. Trigger VoidSpawn

Expected behavior
Allow descent below Y=0

Versions

>version
[11:33:50] [Server thread/INFO]: This server is running CraftBukkit version 3304-Spigot-d5de734-dd02403 (MC: 1.18 Pre-release 5) (Implementing API version 1.18-pre5-R0.1-SNAPSHOT)
[11:33:50] [Server thread/INFO]: You are running the latest version
>version VoidSpawn
[11:34:32] [Server thread/INFO]: VoidSpawn version 1.19.5

[SlimeDog]

offset: 59 should be the default for MC 1.18, but that resolves the issue.

[endercrest]

This is part of Voidspawn 1.20, it now defaults to the world min-height value. As well as adding a new option to more easily control this value.