Create Source Map between original and markdown version

Some applications have information that is shared across the transformed and the original version of the notebook files. For example, on a Merge request, if a comment is added on line 30 of the markdown diff, it should not be placed on line 30 of the raw diff.

Fixing this requires a source map, list that tells for each line where each line of the raw diff is located on the markdown diff, and vice versa.