ANCHOR SCROLL: WIP - feat($anchorScroll): add support for configurable scroll offset

Add support for a configurable scroll offset to $anchorScrollProvider.
The offset is expressed in pixels and can be specified either as a fixed value or as a function that returns the offset it pixels dynamically.
(This is a POC and a WIP: no docs, no tests)

Related to #9368

Closes #2070, #9360
You missed an `isNumber()` check in the end, but other than that the statements are functionally identical.
I tried to micro-optimize here (i.e. adding cheaper checks near the top) and I don't find the result overly complex, but if you think it's not worth it, I can remove it.