vim-patch:9.0.0012: signature files not detected properly (#19172)

Problem:    Signature files not detected properly.
Solution:   Add a function to better detect signature files. (Doug Kearns)
cdbfc6dbab

runtime/lua/vim/filetype.lua

@@ -586,7 +586,6 @@ local extension = {
   c = function(path, bufnr)
     return require('vim.filetype.detect').lpc(bufnr)
   end,
-  sig = 'lprolog',
   lsl = 'lsl',
   lss = 'lss',
   nse = 'lua',
@@ -867,6 +866,9 @@ local extension = {
   end,
   sieve = 'sieve',
   siv = 'sieve',
+  sig = function(path, bufnr)
+    return require('vim.filetype.detect').sig(bufnr)
+  end,
   sil = 'sil',
   sim = 'simula',
   ['s85'] = 'sinda',

runtime/lua/vim/filetype/detect.lua

@@ -922,6 +922,23 @@ function M.rules(path)
   end
 end
 
+-- LambdaProlog and Standard ML signature files
+function M.sig(bufnr)
+  if vim.g.filetype_sig then
+    return vim.g.filetype_sig
+  end
+
+  local line = nextnonblank(bufnr, 1)
+
+  -- LambdaProlog comment or keyword
+  if findany(line, { '^%s*/%*', '^%s*%%', '^%s*sig%s+%a' }) then
+    return 'lprolog'
+    -- SML comment or keyword
+  elseif findany(line, { '^%s*%(%*', '^%s*signature%s+%a', '^%s*structure%s+%a' }) then
+    return 'sml'
+  end
+end
+
 -- This function checks the first 25 lines of file extension "sc" to resolve
 -- detection between scala and SuperCollider
 function M.sc(bufnr)