Use Adventure/MiniMessage

[srnyx]

• Adventure documentation: https://docs.advntr.dev/
• MiniMessage documentation: https://docs.advntr.dev/minimessage

But keep backwards compatibility for the legacy message system

[srnyx]

Superseded by #13

[srnyx]

Reopening because I may still not want to switch to Paper yet. Keeping support for Spigot is still feasible and just overall preferred.