Bug 1425310 - Implement modulepreload for link rel. r=yulia,smaug,jonco

Does perform the optional step of fetching descendants and linking:
https://html.spec.whatwg.org/multipage/webappapis.html#fetching-scripts:fetch-the-descendants-of-and-link-a-module-script-2

Partially implements some new destinations ("as" attribute values) for
modulepreload only (not for preload or link attribute reflection)

Differential Revision: https://phabricator.services.mozilla.com/D172368