feat: Forward ref to ModalBody

[pawelgrimm]

Short description

We are adding ref forwarding to ModalBody. This will allow consumers to gain a reference to the scrollable container within the ModalBody, which might be relevant for things like virtualization.

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Reviewed and approved Chromatic visual regression tests in CI

Versioning

Minor version update for a new, backwards compatible feature.