implement .clone, .slice, and yxml.get

src/types/YXmlText.js

@@ -14,6 +14,15 @@ export class YXmlText extends YText {
     return new YXmlText()
   }
 
+  /**
+   * @return {YXmlText}
+   */
+  clone () {
+    const text = new YXmlText()
+    text.applyDelta(this.toDelta())
+    return text
+  }
+
   /**
    * Creates a Dom Element that mirrors this YXmlText.
    *