[rfd] a new core.whitespace class "\No newline at end of file"

It would be a good sized internship project to introduce a new
core.whitespace class that is about a file that ends in an
incomplete line (it would be way too big as a microproject, so I would
not add the #leftoverbits tag to this one).

The student needs to learn quite a lot of things, including

 - they need to study the diff machinery to learn how existing
   whitespace breakages are reported, in order to figure out how to
   add a new class of whitespace breakage;

 - they need to find a good name for this new class;

 - they need to study the apply machinery to learn how existing
   whitespace breakages are reported or corrected, in order to
   figure out how to add support for this new class.

Thoughts?