New spell: combine 2 cards to form a stronger one, and a new enemy effect: protect

+ Add new spell: card-combine: combine 2 identical cards on hand into a stronger card.

+ Add new spell: Card duplicate: duplicate the lowest level card in your hand.

+ Overkill-skill balance: reduce overkill damage from 100 -> 50

+ Revert price-reduction affect Spell-card on shop. Price reduction effect will not affect spells sold in shop.

+ Curse upgrade: buff cost from 10 -> 7

+ Spell: prosperity: increase Cost increase per use from 4 -> 6. It's too strong.

+ Adjust the target arrow of the card-target spell so it's a bit more clear what cards are being targetted.

+ When the stage-end, all non-boss enemy properly retreat the stages instead of being deal damage equal to their HP. This helps with analytic.

+ Add new enemy-effect: protect: reduce player's damage on other enemies.