Add rules and addRule legacy methods to CSSStyleSheet

[classilla]

This has changed a bit, so we'll need to implement our own Fx45 version from the spec.

https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet/addRule
https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet/rules

[classilla]

Testcase in bleh archive as addrule

[classilla]

sticking