foot.ini: add modeline so vim uses # comments instead of ;

Editors like Vim and Kakoune ship filetypes dosini and ini
respectively. Both use ";" as comment character, which is used for
example by Vim's "gq" command which wraps lines while preserving
comment prefixes.

foot only accepts # comments. I think it's desirable to stick to
a single commenting style (easier to diff configs).

Make Vim accept that by adding modeline to use the "conf"
filetype, which is for "generic Unix config files".  We already
have an Emacs modeline at the top, but Vim seems to ignore it.
Kakoune doesn't have that filetype, but a patch has been
proposed to use # whenever it already occurs in the file, see
https://github.com/mawww/kakoune/pull/4537

foot.ini

@@ -177,3 +177,5 @@
 # select-word=BTN_LEFT-2
 # select-word-whitespace=Control+BTN_LEFT-2
 # select-row=BTN_LEFT-3
+
+# vim: ft=conf