feat(design): add skeletonable public api to design core's public api

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Design core only exposes the DaffSkeletonable interface to the public. daffSkeletonableMixin is not usable by the public.
Fixes: N/A

What is the new behavior?

Update design core's public_api to export the skeletonable public_api instead of only the DaffSkeletonable interface.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information