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-customhr.js

@@ -0,0 +1,26 @@
+/**
+ * @param {Editor} editor
+ */
+function register(editor) {
+    editor.addCommand('InsertHorizontalRule', () => {
+        const hrElem = document.createElement('hr');
+        const cNode = editor.selection.getNode();
+        const {parentNode} = cNode;
+        parentNode.insertBefore(hrElem, cNode);
+    });
+
+    editor.ui.registry.addButton('customhr', {
+        icon: 'horizontal-rule',
+        tooltip: 'Insert horizontal line',
+        onAction() {
+            editor.execCommand('InsertHorizontalRule');
+        },
+    });
+}
+
+/**
+ * @return {register}
+ */
+export function getPlugin() {
+    return register;
+}