LibGUI: Clear undo stack when opening a new document

Author: https://github.com/pakerwreah
Commit: 67537bfc80
Pull-request: https://github.com/SerenityOS/serenity/pull/6868
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -41,6 +41,7 @@ TextDocument::~TextDocument()
 bool TextDocument::set_text(const StringView& text)
 {
     m_client_notifications_enabled = false;
+    m_undo_stack.clear();
     m_spans.clear();
     remove_all_lines();