clear selection on viewport/font resize (#4777)

## Summary of the Pull Request
Match conhost behavior and clear selection on viewport/font resize.

## PR Checklist
* [X] Closes #1165 

## Validation Steps Performed
Retried attached bug repro steps

src/cascadia/TerminalControl/TermControl.cpp

@@ -1499,6 +1499,8 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
             return;
         }
 
+        _terminal->ClearSelection();
+
         // Tell the dx engine that our window is now the new size.
         THROW_IF_FAILED(_renderEngine->SetWindowSize(size));