runtime(fortan): update fortran syntax rules and doc notes
Update to the Fortran 2023 standard. Reorganize some code to reflect the
dropping of dialect support in the previous commit. Minor improvements.
closes: vim/vim#1371271cbe8e17a
Co-authored-by: Ajit-Thakkar <142174202+Ajit-Thakkar@users.noreply.github.com>