mu4e: Enforce use of spaces for indentation

There already is a ".editorconfig" file for cross-editor
configuration, but most Emacs users don't use that, so this also has
to be configured the traditional Emacs way.  (Also I have some doubts
that cross-editor configuration is relevant for Emacs Lisp files.)

.dir-locals.el

@@ -0,0 +1,2 @@
+((emacs-lisp-mode
+  (indent-tabs-mode . nil)))