Add file format guidelines

[albertmink]

Good idea, but I am not sure whether this is checked. In .abap files I observed a lot of no empty lines as most recently in https://github.com/BeckerWdf/abapGit/blob/5959ed6a774beef3080e73312b17b2237707b2c1/src/objects/aff/zcl_abapgit_object_drty.clas.abap. In PR the icon at the end of a file, as this one, shows that there is no empty line.

[mbtools]

it's not checked but will caused diffs. i try to keep an eye on it until abaplint does it automatically but sometimes things slip through.