patch 9.1.0635: filetype: SuperHTML template files not recognized

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: #15355
related: #15367

Signed-off-by: EliSauder <24995216+EliSauder@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/ft.vim

@@ -421,7 +421,6 @@ export def FThtml()
       setf htmlangular
       return
     endif
-
     # Check for XHTML
     if getline(n) =~ '\<DTD\s\+XHTML\s'
       setf xhtml
@@ -432,6 +431,11 @@ export def FThtml()
       setf htmldjango
       return
     endif
+    # Check for SuperHTML
+    if getline(n) =~ '<extend\|<super>'
+        setf superhtml
+        return
+    endif
     n += 1
   endwhile
   setf FALLBACK html