pat-scroll: Implement selector:top

[thet]

Ref: #721

Note:
IIRC, The original issue appeared when injecting content into a site where we want to scroll to the top of the page and not keep the old scroll position.
This problem is actually widely known - e.g. vue-router has a special scrollBehavior option for route-changes for this: https://router.vuejs.org/guide/advanced/scroll-behavior.html
An alternative solution would be to implement the scrolling behavior in pat-inject after the injection was done.
@cornae @pilz

[pilz]

@thet we have some scroll impl in inject already, did you know https://github.com/Patternslib/Patterns/blob/master/src/pat/inject/inject.js#L501 ?

[thet]

@pilz - very nice! Did not know this. Can scroll-top for pat-inject be the default? If so, then this work here might be obsolete - although it's still handy to have the option to just scroll up without any further config.

[cornae]

@thet scroll: top as default for pat-inject would have unoverseable consequences. But… we could reach the same effect once we have global pat-inject. #703 With global pat-inject you would only have to put pat-inject on the body tag with the scroll property set to top.

What pat inject scroll cannot do is animate the scroll and sometimes that's more elegant. The top value is either way still very welcome for pat-scroll.