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-about.js

@@ -0,0 +1,24 @@
+/**
+ * @param {Editor} editor
+ */
+function register(editor) {
+    const aboutDialog = {
+        title: 'About the WYSIWYG Editor',
+        url: window.baseUrl('/help/wysiwyg'),
+    };
+
+    editor.ui.registry.addButton('about', {
+        icon: 'help',
+        tooltip: 'About the editor',
+        onAction() {
+            window.tinymce.activeEditor.windowManager.openUrl(aboutDialog);
+        },
+    });
+}
+
+/**
+ * @return {register}
+ */
+export function getPlugin() {
+    return register;
+}