Add support for mdx files

Closes syl20bnr#11288

CHANGELOG.develop

@@ -2546,6 +2546,8 @@ Other:
 - Change the ~C-n~ and ~C-p~ message keys to their original ~N~ and ~P~ bindings
 - Edit require from =org-notmuch= to =ol-notmuch= as per org-mode update 9.3.1
   (thanks to Loys Ollivier)
+**** Markdown
+- Add support for mdx files via markdown-mode (robbyoconnor)
 **** OCaml
 - Allow initialization without requiring =opam= (thanks to Bernhard Schommer)
 - Fixed misused functions, =merlin= is a package not a layer

layers/+lang/markdown/README.org

@@ -34,6 +34,7 @@ This layer adds markdown support to Spacemacs.
 
 ** Features:
 - markdown files support via [[http://jblevins.org/git/markdown-mode.git/][markdown-mode]]
+- [[https://github.com/mdx-js/mdx][mdx]] file support via [[http://jblevins.org/git/markdown-mode.git/][markdown-mode]]
 - Fast GitHub-flavored live preview via [[https://github.com/blak3mill3r/vmd-mode][vmd-mode]]
 - TOC generation via [[https://github.com/ardumont/markdown-toc][markdown-toc]]
 - Completion of Emojis using [[https://github.com/dunn/company-emoji][company-emoji]] (still needs a way of showing, either

layers/+lang/markdown/packages.el

@@ -53,7 +53,8 @@
   (use-package markdown-mode
     :mode
     (("\\.m[k]d" . markdown-mode)
-     ("\\.mdk" . markdown-mode))
+     ("\\.mdk" . markdown-mode)
+     ("\\.mdx" . markdown-mode))
     :defer t
     :config
     (progn
@@ -160,7 +161,7 @@
         "gl" 'outline-next-visible-heading)
       ;; Promotion, Demotion
       (add-hook 'spacemacs-editing-style-hook
-                'spacemacs//markdown-hjkl-promotion-demotion)
+         'spacemacs//markdown-hjkl-promotion-demotion)
       (spacemacs//markdown-hjkl-promotion-demotion dotspacemacs-editing-style)
       (define-key markdown-mode-map (kbd "M-<down>") 'markdown-move-down)
       (define-key markdown-mode-map (kbd "M-<left>") 'markdown-promote)