Fix vararg indices being unknown when iterated

[PennyJim]

Big thanks to @tomlau10 for basically writing the fix for me.

After some testing and trying any weird thing I could think of to break the fix, it seemed pretty bulletproof, but I wouldn't be surprised if I'm just not creative enough.

I went ahead and added a test and an entry to the changelog.
This is my first time contributing to luals, so I'd love constructive feedback. Like I feel like my wording is bad no matter how I think about wording things 😁

FIXES #2906

[tomlau10]

Hi @PennyJim, you shall also add a line fix #2906 in your github PR comment. (you can just edit the current one)
This can let github to link this PR to close your original issue on merge. 👀
https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue

Similar to this one: #2888, where there is a FIXES #2887 in the PR comment.
Then the PR page will have something like Successfully merging this pull request may close these issues: #2906 on the right hand side of the page 😄

[sumneko]

Thank you!