fix: add ViewportOverlay as a named export (#11)

Adds `ViewportOverlay` as a named export of the package, additionally to the default export of `CornerstoneViewport`.

Explicit export would allow downstream users to import and use `ViewportOverlay` separately in their projects (e.g. in OHIF/Viewers extensions or customized overlays).

src/index.js

@@ -1,4 +1,8 @@
 import './metadataProvider.js';
 import CornerstoneViewport from './CornerstoneViewport/CornerstoneViewport.js';
 
+import ViewportOverlay from './ViewportOverlay/ViewportOverlay';
+
+export { ViewportOverlay };
+
 export default CornerstoneViewport;