revert dynamic Codemirror mode import

- Closes #653

lib/constants.js

@@ -908,8 +908,18 @@ const unfold = (f, seed) => {
 }`
 
 if (typeof window !== 'undefined' && typeof window.navigator !== 'undefined') {
-  require(`codemirror/mode/javascript/javascript`)
-  import('./modes')
+  const alreadyLoaded = new Set()
+
+  LANGUAGES.filter(language => language.mode !== 'auto' && language.mode !== 'text').forEach(
+    language => {
+      if (language.mode && !alreadyLoaded.has(language.mode)) {
+        alreadyLoaded.add(language.mode)
+        language.custom
+          ? require(`./custom/modes/${language.mode}`)
+          : require(`codemirror/mode/${language.mode}/${language.mode}`)
+      }
+    }
+  )
 }
 
 export const DEFAULT_SETTINGS = {