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/lua/vim/filetype.lua

@@ -902,7 +902,9 @@ local extension = {
   sig = function(path, bufnr)
     return require('vim.filetype.detect').sig(bufnr)
   end,
-  sil = 'sil',
+  sil = function(path, bufnr)
+    return require('vim.filetype.detect').sil(bufnr)
+  end,
   sim = 'simula',
   ['s85'] = 'sinda',
   sin = 'sinda',

runtime/lua/vim/filetype/detect.lua

@@ -1194,6 +1194,19 @@ function M.shell(path, contents, name)
   return name
 end
 
+-- Swift Intermediate Language or SILE
+function M.sil(bufnr)
+  for _, line in ipairs(getlines(bufnr, 1, 100)) do
+    if line:find('^%s*[\\%%]') then
+      return 'sile'
+    elseif line:find('^%s*%S') then
+      return 'sil'
+    end
+  end
+  -- No clue, default to "sil"
+  return 'sil'
+end
+
 -- SMIL or SNMP MIB file
 function M.smi(bufnr)
   local line = getlines(bufnr, 1)