Polyfill ShadowRoot.getSelection when not available

FIX: The `root` accessor on views now makes sure that, when it returns a
shadow root, that object has a `getSelection` method.

See https://discuss.prosemirror.net/t/prosemirror-inside-a-shadowroot/2387

src/index.js

@@ -247,8 +247,10 @@ export class EditorView {
   get root() {
     let cached = this._root
     if (cached == null) for (let search = this.dom.parentNode; search; search = search.parentNode) {
-      if (search.nodeType == 9 || (search.nodeType == 11 && search.host))
+      if (search.nodeType == 9 || (search.nodeType == 11 && search.host)) {
+        if (!search.getSelection) Object.getPrototypeOf(search).getSelection = () => document.getSelection()
         return this._root = search
+      }
     }
     return cached || document
   }