[Fortran] submodules

Let's say that I'm editing a module that is very deep in the code base. For example, I might want to experiment with different ways of factorizing a matrix, without changing the signature of the subroutine. Clicking build in Visual Studio will re-compile the module that I'm editing, and then every other that uses it. It's the so-called "compilation cascade", which is a major waste of time. To fix this behavior, the module should be converted to a [sub-module](https://fortranwiki.org/fortran/show/Submodules), which is the Fortran equivalent of header files.

It would be nice if codee could split the module into sub-module and interface automatically. See also the example on [fortran-lang](https://fortran-lang.org/en/learn/best_practices/modules_programs/).