[popover2] Popover2 and ContextMenu2 fixes & enhancements

[adidahiya]

Ported changes on v4 branch from #4588 and #4595 onto develop.

Fixes #4537

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Refactor Tree example as a function component with cleaner state handling logic (using a React reducer hook)
• Add a repro case for ContextMenu2 with Tree - wrong open position #4537 in the refactored Tree example
• Add positioningStrategy prop for Popover2
• Add popoverClassName prop for ContextMenu2
• Fix ContextMenu2's styles to use their own classes instead of ContextMenu's, add the relevant Sass stylesheet
• Create a new utility class .bp3-fixed-positioning-containing-block which is used to give hints to child / ancestor elements which use position: fixed and need to know about their containing block for proper layout calculations. See MDN docs. Use this new utility class in Collapse and ContextMenu2 to fix ContextMenu2 with Tree - wrong open position #4537.
• Update ContextMenu2RenderProps to include the mouse event like ContextMenuTarget did, to improve compatibility with existing use cases in the wild (like I encountered in the table package in [v4] [table] migrate from ContextMenuTarget to ContextMenu2 #4595)
• Add className, disabled, and onContextMenu props for ContextMenu2

Reviewers should focus on:

Already reviewed

[blueprint-bot]

[popover2] Popover2 and ContextMenu2 fixes & enhancements

Previews: documentation | landing | table

[adidahiya]

ContextMenu2 and Tree examples look good in docs preview 👍