[v5.1] Project structure changes + input validation

This update introduces some big changes to the project structure (most noticeably I got rid of the "internal" and "custom" folders and instead moved everything out to the root folder, I thought it made more sense that way)

Also packet handlers and senders are now split into different files! Previously it would get pretty inconvenient to have everything in one place as your project eventually grows in scale

You can still write your handlers and senders as before if you want to though!

In addition to that, I added the ability to validate incoming data using the library "fastest-validator". This is useful for additional type/value safety, as well as for potentially preventing cheaters from hijacking the packets to e.x. submit a really long username or other invalid data that could potentially break the backend and would otherwise be only filtered on the client-side

I know there haven't been any updates in quite a while, so I hope you like this one!
- Evoleo