Fix webpack public path error.

Author: jasongrout

{{cookiecutter.github_project_name}}/src/extension.ts

@@ -8,6 +8,6 @@
 // Some static assets may be required by the custom widget javascript. The base
 // url for the notebook is not known at build time and is therefore computed
 // dynamically.
-__webpack_public_path__ = document.querySelector('body')!.getAttribute('data-base-url') + 'nbextensions/{{ cookiecutter.npm_package_name }}';
+(window as any).__webpack_public_path__ = document.querySelector('body')!.getAttribute('data-base-url') + 'nbextensions/{{ cookiecutter.npm_package_name }}';
 
 export * from './index';