Proposal: Synchronous ContentDialog

The idea is that non WinUI3 apps that would switch to WinUI are using MessageBox and DialogBox which are blocking calls; The caller expects a return value.

Trying to create another message loop doesn't work, it crashes on return:

```
void MainPage::ShowDialogBlocking(winrt::Microsoft::UI::Xaml::Window const& mainWindow)
{
auto dialog = ... // ContentDialog
auto dispatcherQueue = mainWindow.DispatcherQueue();

auto as = dialog.ShowAsync();
as.Completed([&](auto const&, auto)
{
dispatcherQueue.EnqueueEventLoopExit();
});

dispatcherQueue.RunEventLoop();
}
```

If this could work then non WinUI3 apps will have an easier time in transiting to WinUI3.

Note that the following seems to work but I'm not sure it is reliable.

```
void MainPage::ShowDialogBlocking()
{
auto dialog = ... // ContentDialog
ContentDialogResult res;
bool Completed = 0;
auto as = dialog.ShowAsync();
as.Completed([&](auto const& sender, auto)
{
res = sender.as>().GetResults();
Completed = 1;

});

while (!Completed)
{
MSG msg;
if (GetMessage(&msg, 0,0,0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
}
```

Start by reviewing ContentDialog.ShowAsync, DispatcherQueue.RunEventLoop, and the GetMessage loop shown in the issue, along with how ContentDialogResult is retrieved. Determine whether a supported synchronous ContentDialog API can return a result without crashing or introducing unreliable nested message-loop behavior.

Written by the indexing model from the issue text.