fix: fix the default index of Multitool modes

[ybw0014]

Description

Fixed a bug introduced in PR #4122.

Untested, but should be fine with only a small change.

Proposed changes

Set the default index to 0, as PersistentDataAPI.getInt returns -1 by default.

Related Issues (if applicable)

Fixes #4203

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! ❤️

[JustAHuman-xD]

Nice catch

[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: ec3c946d

https://preview-builds.walshy.dev/download/Slimefun/4202/ec3c946d

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!