Introduce yarn workspace, and use beachball for publishing

[acoates-ms]

This gets us most of the way to supporting multiple packages within the RNW repo.

It introduces a yarn workspace which contains the vnext package and a placeholder, "additional" package, and it removes my hand rolled publishing logic in favor of beachball which will be able to handle version bumping and publishing multiple packages together, and will automatically generate changelogs.

For future checkins:

• Introduce additional CI logic verifying that all changes have change files.
• Add learna to orchestrate multi package builds.
• Currently RN/** is marked as nohoist to reduce more churn. -- But for dev productivity we should look at getting that reduced in a future checkin.

Microsoft Reviewers: Open in CodeFlow

[ahimberg]

should current be in here?

[acoates-ms]

No. Current will not be manged by beachball, or yarn. Since current and vnext both have the same package names, it confuses all the monorepo tools.

[ghost]

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