Editors: Added lexical editor for testing

Started basic playground for testing lexical as a new WYSIWYG editor.
Moved out tinymce to be under wysiwyg-tinymce instead so lexical is the
default, but TinyMce code remains.

resources/js/wysiwyg-tinymce/plugins-stub.js

@@ -0,0 +1,15 @@
+/**
+ * @param {Editor} editor
+ * @param {String} url
+ */
+function register(editor, url) {
+
+}
+
+/**
+ * @param {WysiwygConfigOptions} options
+ * @return {register}
+ */
+export function getPlugin(options) {
+    return register;
+}