add ContentAnchor component

[jroebu14]

Resolves #2279

Overall change: Adds a component that can wrap content such as ads to detect and mitigate content shifting. For more info refer to the README.md

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Above is the Storybook story. To test and showcase the component we have here content that loads a new, different sized image every few seconds. The green shows where the browser repainted sections of the screen as a result of the image changing and causing content reflows. You can see from this example that the content within the wrapper will never resize when in view and hide any overflow of the content within. When it is not in view the wrapper and it's content is allowed to resize and the scroll position is adjusted so the user will not experience any unexpected jumps.

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Code changes:

• Makes use IntersectionObserver and ResizeObserver and dynamically loads polyfills for each if the browser has no support for these APIs
• Polyfills used are:
  • w3c/IntersectionObserver - https://github.com/w3c/IntersectionObserver.
  • ResizeObserver - https://github.com/juggle/resize-observer
• If scroll anchoring isn't supported (IE, Safari) then javascript is used to adjust the scroll position when the content resizes.

NB

• This component needs a better name! Suggestion welcome.
• Ideas for more test cases inside of Storybook are welcome.
• This is the first time I've used dynamic imports and I haven't seen it used in Simorgh or Psammead so maybe we need to have a discussion about it's use.
• Needs an a11y swarm
• IntersectionObserver is polyfilled whereas ResizeObserver is ponyfilled. There's nothing strategic about this, it's just the way these modules were designed. One view is that ponyfills should be preferred over polyfills. I'm not totally convinced but if anyone feels strongly about it I could search for an IntersectionObserver ponyfill
• Writing good unit tests for this component was a challenge. Open to ideas to make them better. I don't think this can be e2e tested until it is implemented in Simorgh(?).
• Unit tests are minimised in the file changed tab. Make sure to expand and have a look.
• Happy to set up a swarm review for this if people feel we need it.

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• I have assigned myself to this PR and the corresponding issues
• Automated jest tests added (for new features) or updated (for existing features)
• This PR requires manual testing

Testing notes
Needs tested in all browsers. Can be tested in Storybook.