Bug 1578564 - Workaround a prettier warning so that we can run it on html/xhtml files. r=vporof

Differential Revision: https://phabricator.services.mozilla.com/D44555

--HG--
extra : moz-landing-system : lando

.prettierrc

@@ -1,5 +1,9 @@
 {
   "printWidth": 80,
   "tabWidth": 2,
-  "trailingComma": "es5"
+  "trailingComma": "es5",
+  "overrides": [{
+    "files": ["*.html", "*.xhtml"],
+    "options": { "parser": "babel" }
+  }]
 }