Continuing to work through and complete requests from player feedback, as well as adding more content

- undo button: press F5 to revert to start of turn if you make a mistake

- added feedback for hurting/killing multiple enemies in one action

- added damage multiplier card - discard a card to double the damage on your next 2 attacks

- added sneak card - enemies will not notice your movement and head towards your previous position, until you attack or use a non-sneak movement card

- buffed movement distance

- fixed card selection/movement click bug

- added some music

- added more obvious indications for when discarding cards

- minor UI tweaks