HackStudio: Stop crash from leaking Editor widgets when closing tabs

When re-opening an existing file, we would reuse the document and
register a new Editor with it, but never unregister that Editor.

Previously, this would cause a crash if you opened a binary file, closed
its tab, then opened that binary file again. HackStudio would crash
while calling `HackStudio::EditorWrapper::update_title()` on an invalid
EditorWrapper. But now it doesn't!

Something still gets leaked each time, but we now don't crash at least.

Userland/DevTools/HackStudio/HackStudioWidget.cpp

@@ -801,6 +801,7 @@ void HackStudioWidget::add_new_editor(GUI::TabWidget& parent)
 
  wrapper.on_tab_close_request = [this, &parent](auto& tab) {
  parent.deferred_invoke([this, &parent, &tab] {
+ tab.editor().document().unregister_client(tab.editor());
  set_current_editor_wrapper(tab);
  parent.remove_tab(tab);
  m_all_editor_wrappers.remove_first_matching([&tab](auto& entry) { return entry == &tab; });