feat($anchorScroll): support a configurable vertical scroll offset

Add support for a configurable vertical scroll offset to `$anchorScroll`.

The offset can be defined by a specific number of pixels, a callback function
that returns the number of pixels on demand or a jqLite/JQuery wrapped DOM
element whose height and position are used if it has fixed position.

The offset algorithm takes into account items that are near the bottom of
the page preventing over-zealous offset correction.

Closes #9368
Closes #2070
Closes #9360