Handle native undo/redo

Currently our editor listens on the common hotkeys (cmd+z / cmd+shift+z)
for undo/redo but the user could always undo using native browser UI
like right-click, the edit topbar menu, and on mobile iOS there is a toolbar,
and the native undo usually does not do the right thing.

Most modern browsers support `beforeinput` to interrupt native undo/redo inputs
so we'll use this to handle them natively.

modules/history.js

@@ -33,6 +33,16 @@ class History extends Module {
         this.redo.bind(this),
       );
     }
+
+    this.quill.root.addEventListener('beforeinput', event => {
+      if (event.inputType === 'historyUndo') {
+        this.undo();
+        event.preventDefault();
+      } else if (event.inputType === 'historyRedo') {
+        this.redo();
+        event.preventDefault();
+      }
+    });
   }
 
   change(source, dest) {