Add tests for typewiz-webpack

[urish]

We can run Webpack with a configuration similar to the one in the README file, and then compare the collected types json file with a reference one.

We can probably even run the entire test inside node (using some polyfill for the XHR part), or we can run it inside a real browser using puppeteer.

[zoehneto]

For testing the loader portion of typewiz-webpack the webpack guide for loader testing should explain all that is needed.