Problem: filetype: SuperHTML template files not recognized
Solution: Update the filetype detection code to detect '*.shtml' either
as HTML (Server Side Includes) or SuperHTML (template files)
(EliSauder)
related: vim/vim#15355
related: vim/vim#15367e57c9a19ed
Co-authored-by: EliSauder <24995216+EliSauder@users.noreply.github.com>