vim: use :setf to set the filetype (#8914)

This is nicer because it only sets the filetype if it hasn't already
been set. :setf[iletype] has been available since vim version 6.

See: https://vimhelp.org/options.txt.html#%3Asetf

src/config/vim.zig

@@ -10,7 +10,7 @@ pub const ftdetect =
     \\"
     \\" THIS FILE IS AUTO-GENERATED
     \\
-    \\au BufRead,BufNewFile */ghostty/config,*/ghostty/themes/* set ft=ghostty
+    \\au BufRead,BufNewFile */ghostty/config,*/ghostty/themes/* setf ghostty
     \\
 ;
 pub const ftplugin =