Support interoperableCSS properly

For existing webpack apps we should explore whether we can build an iCSS interface to css-blocks that works well. We had an early implementation of this, but I am no longer confident that it works sanely and I think re-implementing it from scratch on top of our rewriter code (or a common shared implementation) would be for the best.

Start by locating the early iCSS implementation and the existing rewriter code mentioned in the issue. Determine how an iCSS interface should work for existing webpack apps, then confirm that the replacement behaves sanely and can use the rewriter or a shared implementation.

Written by the indexing model from the issue text.