feat: MenuItem2 component

[adidahiya]

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Create new MenuItem2 component in @blueprintjs/popover2, a replacement for MenuItem which uses Popover2 instead of Popover to render submenus
  • Note: it would be more convenient (and less of an upgrade burden on consumers) if we could only deprecate usage of MenuItem with children defined (those which have submenus), but we cannot do that with TypeScript's JSX type checking features, so we must deprecate MenuItem completely to be safe.
• Deprecate MenuItem
• Migrate MenuItem -> MenuItem2 across all non-deprecated Blueprint components in this repo

Reviewers should focus on:

No regressions in MenuItem2 docs example compared to MenuItem

Screenshot

MenuItem2 example:

Updated Popover2 example:

Updated Select2 example:

Updated icon docs page:

Updated ContextMenu2 example:

Updated table example:

[blueprint-bot]

MenuItem2 component, part 4: migrate to MenuItem2 within repo

Previews: documentation | landing | table | demo

[blueprint-bot]

fix unit tests

Previews: documentation | landing | table | demo

[adidahiya]

Note that this is a small breaking change, but in practice I think it should be ok for consumers since I didn't see any usage of popoverProps: Partial<IPopoverProps> for this component in any of our code bases.