Refactor shield activation and always trigger ShieldSpecial when pressed Shield and not turning super

This lets the ShieldSpecial hook work more like the other dedicated button hooks, allowing modders to easily put custom behaviour on the Shield button. Spinshieldhack will use the old behaviour though.

Similar to JumpSpinSpecial, turning super has priority over this hook. That's a job for the SuperReady hook if that ever gets merged lol.