emmet-mode: bind to TAB & enable in reason-mode

TAB now either indents, expands the snippet at point, or expands the
emmet expression at point.

Also it now falls back to emmet-expand-line if yasnippet won't load.

modules/lang/web/autoload/html.el

@@ -105,3 +105,18 @@ function @ http://ergoemacs.org/emacs/elisp_replace_html_entities_command.html"
   "Decode HTML entities in region."
   (interactive "r")
   (+web--entities-region beg end t))
+
+;;;###autoload
+(defun +web/indent-or-yas-or-emmet-expand ()
+  "Invoke `indent-for-tab-command' if at or before text bol, `yas-expand' if on
+a snippet, or `emmet-expand-yas'/`emmet-expand-line', depending on whether
+`yas-minor-mode' is enabled or not."
+  (interactive)
+  (call-interactively
+   (cond ((<= (current-column) (current-indentation))
+          #'indent-for-tab-command)
+         ((bound-and-true-p yas-minor-mode)
+          (if (yas--templates-for-key-at-point)
+              #'yas-expand
+            #'emmet-expand-yas))
+         (#'emmet-expand-line))))

modules/lang/web/config.el

@@ -16,14 +16,15 @@
 (def-package! emmet-mode
   :commands emmet-mode
   :preface (defvar emmet-mode-keymap (make-sparse-keymap))
-  :hook (css-mode web-mode html-mode haml-mode nxml-mode rjsx-mode)
+  :hook (css-mode web-mode html-mode haml-mode nxml-mode rjsx-mode reason-mode)
   :config
+  (when (require 'yasnippet nil t)
+    (add-hook 'emmet-mode-hook #'yas-minor-mode-on))
   (setq emmet-move-cursor-between-quotes t)
-  (add-hook! 'rjsx-mode-hook
-    (setq-local emmet-expand-jsx-className? t))
+  (setq-hook! 'rjsx-mode-hook emmet-expand-jsx-className? t)
   (map! :map emmet-mode-keymap
-        :v "M-e" #'emmet-wrap-with-markup
-        :i "M-e" #'emmet-expand-yas
+        :v [tab] #'emmet-wrap-with-markup
+        :i [tab] #'+web/indent-or-yas-or-emmet-expand
         :i "M-E" #'emmet-expand-line))