Support using trusted types for supplying the script URL

[luser]

#104 raised a valid feature request: users should be able to pass a TrustedScriptURL for the script path. This type is part of the Trusted Types API, a spec that includes types to perform additional validation to help prevent XSS attacks. To usefully support this, we will need to support it in the web capture API. Once that happens, we can expose it via this SDK.

Thanks for bringing this to our attention, @antifuchs!

[timtischler]

+1!! I've installed FS many times at different places with zero hassle but the site I'm trying to install it on now already uses the trusted types api and I haven't figured out to make it work yet. I've spent more time installing it today than the last 3 times I've used it!