LibMarkdown: Make Text default-constructible and move-assignable

Author: https://github.com/alimpfard
Commit: 445bd86533
Pull-request: https://github.com/SerenityOS/serenity/pull/3562
Reviewed-by: https://github.com/bugaevc

Libraries/LibMarkdown/Text.h

@@ -50,6 +50,9 @@ public:
     };
 
     Text(Text&& text) = default;
+    Text() = default;
+
+    Text& operator=(Text&&) = default;
 
     const Vector<Span>& spans() const { return m_spans; }