Allow control over message bar value

[thatblindgeye]

Per a demo built in #794, we should allow control over the underlying input value when the value prop is passed.

[github-actions]

🎉 This issue has been resolved in version 6.5.0-prerelease.30 🎉

The release is available on:

• GitHub release
• npm package (@prerelease dist-tag)

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