Use markdown-mode for mdx files

There's no actual language mode for mdx files yet:
mdx-js/mdx#119

Author: jeffkreeftmeijer

default.el

@@ -144,7 +144,8 @@
 (use-package markdown-mode
   :config (message "Loaded markdown-mode")
   :defer t
-  :ensure t)
+  :ensure t
+  :mode ("\\.mdx\\'" . markdown-mode))
 
 (use-package emacs
   :custom

emacs-configuration.org

@@ -372,7 +372,8 @@ To ensure the packages aren't loaded before they're needed, a message is added t
   (use-package markdown-mode
     :config (message "Loaded markdown-mode")
     :defer t
-    :ensure t)
+    :ensure t
+    :mode ("\\.mdx\\'" . markdown-mode))
 #+end_src
 
 * Org-babel