chore(mixins): add private package for initial implementation of RFC 814

[mrgrain]

Reason for this change

Initial setup of RFC 814 mixins functionality as a private preview package for development and testing.

Description of changes

Added a new private package @aws-cdk/mixins-preview with stub implementation for the mixins pattern described in RFC 814. This establishes the package structure and allows for iterative development of the mixins functionality before public release.

Describe any new or updated permissions being added

No new IAM permissions required.

Description of how you validated changes

Package structure follows CDK conventions and builds successfully.

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

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