Add sneak mode

Add a sneak feature - the player can press the crouch button (B on Xbox controller, O / Circle on Playstation controller) and they will 'crouch'. The 'eye' symbol appears on screen, it's open when the player is not hidden, and a closed eye when the player is hidden. Hiding can be achieved by being outside of the line of sight of enemies or NPC characters. Hiding can also be achieved by being in dark areas/shadow areas and not too close to NPC's or enemies. Players can also attack while in stealth mode and these attacks do bonus damage (show this as text on the screen in the top left corner "Sneak attack for 1.5x damage").