Implement diffToChanges() – a function which converts diff() output into a applicable deltas

[Reinmar]

diff() outputs results like:

diff( 'aba', 'acca' ); // [ EQUAL, INSERT, INSERT, DELETE, EQUAL ]

Often, you may need to then generate a set of deltas which transform the left text (current) to the right one (future) or the opposite. However, the output from diffToChanges() should be generic enough (if possible), so it can be used with JS strings or DOM text nodes in a similar way if needed.

First feature which requires this functionality is ckeditor/ckeditor5-typing. I'm not sure there will be more soon, but that's enough.