refactor(core): add @storybook/ui to typings

We need to "fake" `@storybook/ui` types to be able to compile properly
`@storybook/core`. This is due to a dependency cycle causing `core` to be build at the same time as `ui`:
> lerna WARN ECYCLE Dependency cycles detected, you should fix these!
> lerna WARN ECYCLE @storybook/react -> @storybook/core -> @storybook/ui -> @storybook/react

lib/core/src/typings.d.ts

@@ -7,3 +7,4 @@ declare module '@storybook/theming/paths';
 declare module '@storybook/ui/paths';
 declare module 'file-system-cache';
 declare module 'better-opn';
+declare module '@storybook/ui';