add create export for lib/theming, proper

[ndelangen]

This is actually kind of a good idea, because of the size of lib/theming/index

[yannbf]

Is this a breaking change? If not, is this preferred over import { create } from '@storybook/theming'?

[ndelangen]

I'm effectively reverting the breaking change @yannbf
Yes this is preferred over importing it from the main entry, especially in the manager.ts file.