Resize the drag bar on WM_DISPLAYCHANGE (#4829)

## Summary of the Pull Request

Pretty straightforward. When we get a `WM_DISPLAYCHANGE`, that means the
display's DPI changed. When that happens, resize the drag bar, so that
it'll reflect the new scaling.

Unblocks #4778 
Closes #4166

## Validation
Man I've changed the DPI of my displays so many times in the last 30
minutes. I dragged the window across a bunch of DPI boundaries too.

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -480,6 +480,11 @@ void NonClientIslandWindow::_UpdateFrameMargins() const noexcept
 {
     switch (message)
     {
+    case WM_DISPLAYCHANGE:
+        // GH#4166: When the DPI of the monitor changes out from underneath us,
+        // resize our drag bar, to reflect its newly scaled size.
+        _UpdateIslandRegion();
+        return 0;
     case WM_NCCALCSIZE:
         return _OnNcCalcSize(wParam, lParam);
     case WM_NCHITTEST: