Fix backpack IDs not incrementing

[WalshyDev]

Description

Fixes backpack IDs not incrementing unless the server is closed.

Proposed changes

The ID increment code was still reading the config file, updated this to rely on PlayerData. Made the next ID now be the backpack size. Created a new test to validate behaviour.

Related Issues (if applicable)

Fixes #4080

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

Your Pull Request was automatically labelled as: "✨ Fix"
Thank you for contributing to this project! ❤️

[sonarcloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

9 New issues
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 1953aa09

https://preview-builds.walshy.dev/download/Slimefun/4081/1953aa09

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!