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
This set of if statements seems overly complex...

Why not the following? Am I missing some subtlety?

```js
if if(isElement(offset)) {
  ...
else if (isFunction(offset)) {
 ...
} else  {
  offset = offset || 0;
}
```