Add the HTML focusgroup attribute

[Schweinepriester]

• Spec: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Focusgroup/explainer.md
• Chrome:
  • https://chromestatus.com/feature/5637601087193088
  • https://bugs.chromium.org/p/chromium/issues/detail?id=1286127

Probably a bit early to add currently, but keeping track… :)

[Malvoz]

Polyfill: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Focusgroup/focusgroup_polyfill.js

Edit: focusgroup was moved to OpenUI. Explainer now at: https://open-ui.org/components/focusgroup.explainer