Add GetPropertyPriority to CSSStyleDeclaration

Implementation of #4432 adding `getPropertyPriority` to CSSStyleDeclaration. This is my first attempt at a Servo PR so I'm sure I've done something wrong. Let me know, and I'll fix it up.

As stated in #4432 tests for this are in #4085. If there are additional tests I can write now I would love to do that, I'm just not sure where or what those would be.