fix: command permission handling

[smuu]

Description

This PR introduced the following:

• When registering a new command, one must specify the permission level
• If a player tries to execute a command without having permission, it will show that to the player
• When a player wants to autocomplete a command without having permission, it will not show in the autocomplete

Closes #424

Testing

Checklist

Things need to be done before this Pull Request can be merged.

• Code is well-formatted and adheres to project style guidelines: cargo fmt
• Code does not produce any clippy warnings: cargo clippy
• All unit tests pass: cargo test
• I added new unit tests, so other people don't accidentally break my code by changing other parts of the codebase. How?

[Snowiiii]

Works great, Great work. Thanks for your first Pull request @smuu 🚀