Add EquipmentSlot#getOppositeHand (#10632)

patches/api/Add-EquipmentSlot-convenience-methods.patch

@@ -24,6 +24,21 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +    }
 +
 +    /**
++     * Gets the opposite hand
++     *
++     * @return the opposite hand
++     * @throws IllegalArgumentException if this equipment slot is not a hand
++     * @see #isHand()
++     */
++    public @NotNull EquipmentSlot getOppositeHand() {
++        return switch (this) {
++            case HAND -> OFF_HAND;
++            case OFF_HAND -> HAND;
++            default -> throw new IllegalArgumentException("Unable to determine an opposite hand for equipment slot: " + name());
++        };
++    }
++
++    /**
 +     * Checks whether this equipment slot
 +     * is one of the armor slots:
 +     * {@link #HEAD}, {@link #CHEST},