feat(jqLite): add private jqDocumentComplete function

This helper function can be used to execute a callback only after the
document has completed its loading, i.e. after the `load` event fires
or immediately if the page has already loaded and
`document.readyState === 'complete'`.
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
chore(docs): apply the anchorScroll offset feature to the docs app

Closes #9360
docs(guide/unit-testing): use whitespace to improve readability

☆.。.：・゜☆ Merci (◜௰◝) ☆.。.：・゜☆

Closes #9572
refactor($interpolate): cleanup switch statement, curlies unnecessary

Closes #9530