Expose clean method inside sheet-manager

[minnieliu]

I would like the clean() method inside sheet-manager to be accessible publicly.

Users should be able to clean the registry inside sheet-manager on demand. Useful for server side rendering use case to ensure that class names match between server and client.

Call this function:

[nicholasrice]

I think we can make the registry public

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[chrisdholt]

closed with #3517