[dev-addons] Intent to ship: userScripts extensions API

[Tomislav Jovanovic]

In bug 1514809, I recently enabled the new browser.userScripts
API by default for all channels. It was developed behind the
`extensions.webextensions.userScripts.enabled` pref, with most
of the work done by Luca Greco.

The feature will 🚢 in the next Firefox 68 Beta this Friday,
and reach release in July. Until more detailed documentation
is available on MDN, check out this introduction:

https://blog.mozilla.org/addons/2019/03/26/extensions-in-firefox-67/#userscripts

If you find any issues, please file a bug and link it to:
https://bugzilla.mozilla.org/show_bug.cgi?id=1437098

Cheers
Tom