[markdown] Add support for mdx files

Closes #11288

CHANGELOG.develop

@@ -2454,6 +2454,7 @@ Other:
 - Bound major mode leader key and ~M-RET~ to =markdown-insert-list-item= for
   terminal users (thanks to AmaiKinono)
 - Made =*lsp-help*= buffer shown in a popup window using popwin
+- Add support for mdx files via markdown-mode (thanks to robbyoconnor)
 **** Major modes
 - Added SPARQL-mode (thanks to Dietrich Daroch)
 - Added android logcat (thanks to jiejingzhang)

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)