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

src/nvim/testdir/test_filetype.vim

@@ -1824,6 +1824,44 @@ func Test_sig_file()
   filetype off
 endfunc
 
+" Test dist#ft#FTsil()
+func Test_sil_file()
+  filetype on
+
+  split Xfile.sil
+  call assert_equal('sil', &filetype)
+  bwipe!
+
+  let lines =<< trim END
+  // valid
+  let protoErasedPathA = \ABCProtocol.a
+
+  // also valid
+  let protoErasedPathA =
+          \ABCProtocol.a
+  END
+  call writefile(lines, 'Xfile.sil')
+
+  split Xfile.sil
+  call assert_equal('sil', &filetype)
+  bwipe!
+
+  " SILE
+
+  call writefile(['% some comment'], 'Xfile.sil')
+  split Xfile.sil
+  call assert_equal('sile', &filetype)
+  bwipe!
+
+  call writefile(['\begin[papersize=a6]{document}foo\end{document}'], 'Xfile.sil')
+  split Xfile.sil
+  call assert_equal('sile', &filetype)
+  bwipe!
+
+  call delete('Xfile.sil')
+  filetype off
+endfunc
+
 func Test_inc_file()
   filetype on