Intent to Prototype and Ship: IntersectionObserver scrollMargin

Summary:

  Adds the `scrollMargin` property to IntersectionObserverInit, which factors into the intersection calculations.

Bug:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1860030

Specification:

  https://w3c.github.io/IntersectionObserver/#dom-intersectionobserver-scrollmargin

Standards Body:

  W3C

Platform coverage:

  all.

Preference:

  dom.intersection_observer.scroll_margin.enabled

DevTools bug:

  N/A

Link to standards-positions discussion:

  N/A

Other browsers:

  Blink: Implemented & Shipped in M120 (https://chromestatus.com/feature/5091020593430528).

  WebKit: Positive (https://github.com/w3c/IntersectionObserver/issues/431#issuecomment-930602435).

web-platform-tests:

  https://wpt.fyi/results/intersection-observer?label=master&label=experimental&aligned&q=scroll-margin.

Over the coming weeks I intend to implement the `scrollMargin` property on IntersectionObserver. I aim to have this ready and enabled for the 141 milestone (June 19 soft freeze). 

Worth pointing out: using scroll margin for lazy loading is a separate feature (tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1864794) which is dependent on this feature, but will ship independently of this.