[Bug fortran/93736] Add .f18 and .F18 file suffixes

[thenlich at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93736

--- Comment #7 from Thomas Henlich <thenlich at gcc dot gnu.org> ---
This should have a test-case added, ideally by renaming an already existing
test-case containing Fortran 2018 features to .f18.