fix(filetype): set default ft_ignore_pat in filetype.lua (#16917)

This default value is also set in filetype.vim, but if filetype.vim is
disabled the variable is never defined, which causes errors in some of
the dist#ft detection functions.

runtime/filetype.lua

@@ -20,3 +20,7 @@ let g:did_load_ftdetect = 1
 
 augroup END
 ]]
+
+if not vim.g.ft_ignore_pat then
+  vim.g.ft_ignore_pat = "\\.\\(Z\\|gz\\|bz2\\|zip\\|tgz\\)$"
+end