[Not Release] [JS] Implement array grouping proposal

[Rumyra]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
  or
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

Array.prototype.groupBy and Array.prototype.groupByMap

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1739648

MDN URL: https://developer.mozilla.org/en-US/docs/Web/JavaScript

[hamishwillee]

This is not part of the release as the change is going into nightly only.
Status:

• BCD - FF98 Array.groupBy()/.groupByToMap() browser-compat-data#14965
• Docs
  • Array.groupBy() - Array.prototype.groupBy - add #12980
  • Array.groupByToMap() - Add docs Array.groupByToMap #13171
  • Array - done as part of above
• Experimental features > JavaScript Add docs Array.groupByToMap #13171
• Interactive examples
  • groupBy: Array.groupBy() example interactive-examples#2014
  • groupByToMap: Map.groupBy() interactive example interactive-examples#2015
• dev-docs-complete

[Josh-Cena]

@hamishwillee IIUC, this is only waiting for the feature to go stable, right? (Taking much longer than expected)