Customize: Add support for previewing requests made via the Fetch API

[westonruter]

The Customizer preview supports applying the customized state to Ajax requests via jQuery.prefilterAjax. However, if a frontend is not using jQuery but rather the Fetch API then the customized state will not be injected into the request in order to have the customized state applied to the response. In the same way that customize-preview.js is wrapping history.replaceState() and history.pushState() to inject the customized state, we should also wrap fetch() to do the same. We may also want to do this for XMLHttpRequest as well.

[westonruter]

Feature plugin for testing: ​https://github.com/xwp/wp-customize-preview-fetch-api

Pull requests welcome. There are some outstanding todos for supporting the passing of Request into fetch() as well as handling of Headers.

[westonruter]

New feature development is being directed toward the Site Editor, so I'm closing this.