Following a high speed chase, plaintiffs stopped, then made a three-point turn in a controlled manner and their vehicle was moving slowly forward, not accelerating and not pointed directly at officers; officers who fired upon them “seized” both driver and passenger; court rejects officers’ claim that passenger was not seized because they intended to shoot only driver, because stopping car would have seized passenger; construing facts in favor of plaintiffs, seizure of slowly moving car was unreasonable where officers could have easily stepped out of vehicle’s path to avoid danger; officers