Storybook Core-Common

Common utilities used across @storybook/core-server (manager UI configuration) and @storybook/builder-webpack{4,5} (preview configuration).

This is a lot of code extracted for convenience, not because it made sense.

Supporting multiple version of webpack and this duplicating a large portion of code that was never meant to be generic caused this.

At some point we'll refactor this, it's likely a lot of this code is dead or barely used.