vim-patch:9.0.1587: Corn config files are not recognized (#23807)

Problem:    Corn config files are not recognized.
Solution:   Add a pattern for Corn config files. (Jake Stanger, closes vim/vim#12449)

05843e8960

Co-authored-by: Jake Stanger <mail@jstanger.dev>

runtime/lua/vim/filetype.lua

@@ -241,6 +241,7 @@ local extension = {
   copyright = function(path, bufnr)
     return require('vim.filetype.detect').copyright(bufnr)
   end,
+  corn = 'corn',
   csh = function(path, bufnr)
     return require('vim.filetype.detect').csh(path, bufnr)
   end,