Update nyxx_lavalink to use nyxx 6.0.0 and Lavalink v4

[abitofevrything]

Description

This PR is a complete rewrite of nyxx_lavalink that updates nyxx to 6.0.0 and Lavalink to v4. It uses nyxx's plugin system, simplifies the Lavalink connection logic and harmonises the API to be more similar to the 6.0.0 nyxx API.

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works