vim-patch:9.0.0349: filetype of *.sil files not well detected (#20050)

Problem:    Filetype of *.sil files not well detected.
Solution:   Inspect the file contents to guess the filetype.
be807d5824

runtime/autoload/dist/ft.vim

@@ -468,7 +468,7 @@ endfunc
 
 " Returns true if file content looks like LambdaProlog module
 func IsLProlog()
-  " skip apparent comments and blank lines, what looks like 
+  " skip apparent comments and blank lines, what looks like
   " LambdaProlog comment may be RAPID header
   let l = nextnonblank(1)
   while l > 0 && l < line('$') && getline(l) =~ '^\s*%' " LambdaProlog comment
@@ -877,6 +877,23 @@ func dist#ft#FTsig()
   endif
 endfunc
 
+" This function checks the first 100 lines of files matching "*.sil" to
+" resolve detection between Swift Intermediate Language and SILE.
+func dist#ft#FTsil()
+  for lnum in range(1, [line('$'), 100]->min())
+    let line = getline(lnum)
+    if line =~ '^\s*[\\%]'
+      setf sile
+      return
+    elseif line =~ '^\s*\S'
+      setf sil
+      return
+    endif
+  endfor
+  " no clue, default to "sil"
+  setf sil
+endfunc
+
 func dist#ft#FTsys()
   if exists("g:filetype_sys")
     exe "setf " .. g:filetype_sys