runtime(fortran): update fortran syntax (vim/vim#13870)
Support most remaining features of Fortran 2018/2023
Small improvements to folding etc,
Code cleanup: use \? instead of mix of \= and \?
ef79c57837
Co-authored-by: Ajit-Thakkar <142174202+Ajit-Thakkar@users.noreply.github.com>