Support defer attribute of the script tag

[Frizlab]

Script support .asynchronously() for async, but does not support defer AFAICT.
(The obvious workaround of using .custom(key: "defer", value: "defer") works, but having first-class support would be better IMHO.)

[mattesmohr]

You are right. The standard lists the attribute for Script. It seems, I have missed that. Thanks for reporting. I will add it, tonight.