Use source.fortran.modern TM scope for FORTRAN

This is technically only for FORTRAN 90 and newer, but seems to do just fine with older variants.
Merge pull request #1708 from github/fortran-modern

Use source.fortran.modern TM scope for FORTRAN
Add assert for Emacs lisp

/cc https://github.com/github/linguist/pull/1499
Merge pull request #1709 from github/emacs-lisp-assertion

Add assertion for Emacs Lisp
Add FORTRAN samples.
Add Forth extensions .f and .for; add heuristics for Forth and FORTRAN.