Add a getter and setter for Player.heldSlot

[AkmalFairuz]

This pull request introduces new functionality for managing the player's held slot in the hotbar and includes validation to ensure the selected slot is within the valid range. The most important changes include the addition of new methods to the Player struct and updates to the Session struct to handle slot validation.

New functionality for managing held slots:

• server/player/player.go: Added HeldSlot and SetHeldSlot methods to the Player struct for getting and setting the currently selected hotbar slot.

Validation improvements:

• server/session/player.go: Updated the SetHeldSlot and UpdateHeldSlot method in the Session struct to include validation for the slot range, ensuring it is between 0 and 8.

[TwistedAsylumMC]

Closing this in favour of #585, still pending further discussion before being merged. We appreciate the pull request though!