feat(mixins-preview): developer preview of CDK Mixins

[mrgrain]

Reason for this change

aws/aws-cdk-rfcs#824

CDK Mixins are composable, reusable abstractions that can be applied to any construct (L1, L2 or custom).
They are breaking down the traditional barriers between construct levels,
allowing customers to mix and match sophisticated features without being locked into specific implementations.

Description of changes

This PR makes the package public so it can be released. It also implements some small changes based on RFC feedback.
Main functional changes are:

• Removing validate() in favor of just throwing errors
• Making .with(), .apply() and .mustApply() variadic

Describe any new or updated permissions being added

n/a

Description of how you validated changes

Unit tests.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

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