Is there a plugin/script that allows me to create keyframes based on the stereo field of the audio?

SoundKeys will let you focus on specific frequency ranges, but if all you need is left and right amplitude keyframes, AE's Animation > Keyframe Assistant > Convert Audio to Keyframes will give you keyframes for Left, Right, and Both channels. 

Sound Keys, part of the Red Giant Suite from Maxon, is the tool you want.

I don't know of a stand-alone version or Sound Keys or another similar effect available as a single product.