Re-add extensions as top level dir

extensions/ohif-dicom-pdf-extension/src/OHIFComponentPlugin.js

@@ -0,0 +1,15 @@
+import React from 'react';
+import PropTypes from "prop-types";
+
+function OHIFComponentPlugin(props) {
+  return (<React.Fragment>{props.children}</React.Fragment>);
+}
+
+OHIFComponentPlugin.propTypes = {
+  id: PropTypes.string.isRequired,
+  init: PropTypes.func.isRequired,
+  destroy: PropTypes.func.isRequired,
+  children: PropTypes.node
+};
+
+export default OHIFComponentPlugin;