modify nametag_visibility in arena.yml + fixed teamstats: false when using team

[HMS-CV-Unicorn]

This is my first pull request.
I appreciate your patience and understanding as I navigate this process.

Changes Made:
1.Nametag Visibility Customization
I added the ability to set the nametag_visibility property in arena.yml when using the team_colors module.
This is useful for scenarios where you want to hide other teams' name tags from view.

2.Individual vs. Team Victory Setting
I noticed that even when Teamstats: false was set, all team members were declared as winners. I fixed this issue so that the plugin now correctly supports individual victories.
This makes it easier to configure Free-For-All (FFA) arenas. By setting everyone on the same team, hiding their name tags with nametag_visibility, and enabling friendly fire, FFA setups can be achieved effortlessly.

[Redned235]

Thanks for the PR! Overall, looks pretty good, however I'd double check the spacing since it seems there's a few issues there. IntelliJ should be able to resolve this (see: https://www.jetbrains.com/help/idea/reformat-and-rearrange-code.html).