Adds generic impact noises to all projectiles

Adds a generic projectile hit sound (pierce.ogg, or alien stun hands) to all projectiles but practice laser and laser tag beams. Overrides the sound with specific ones for electrodes and lasers: sear.ogg for lasers and "sparks" (AKA stun baton turning on) for electrodes. Scales volumes of projectile hit sounds based on damage done by the projectile.

Thanks @optimumtact for your help with getting the hitsound volume scaling to work.