Using the subscript loader is currently our best way of getting the loaded scripts recorded for entry
in the ScriptPreloader which is used to cache startup-related JavaScript within the startup window.
Originally, we were using <script> tags to load those scripts, but unfortunately, that loading mechanism
does not qualify for being noted by the ScriptPreloader.
This is a workaround until the Stencil project by the SpiderMonkey team Phase 2 is complete (see bug 1663956),
which should allow us to more easily note scripts to cache at runtime.
Differential Revision: https://phabricator.services.mozilla.com/D95407
