Move prettier-loader to devDependencies

[kellyi]

Overview

Having prettier-loader under dependencies leads to this warning when using the library in another application:

It's non-fatal and probably mostly inconsequential but just to be sure we should move the prettier-loader to devDependencies.

Expected Behavior

prettier-loader is in devDependencies & prettier-loader still runs automatically on file system changes.

Actual Behavior

prettier-loader is in dependencies