Extract extensions from frontend-dynamic-style-sheet into a separate module

[andrew-k-21-12]

The extensions package from the frontend-dynamic-style-sheet module contains only quite small and independent extensions. Consider extracting them into a separate utility module.

The biggest question here is whether to treat ElementType<P>.invoke extensions as a part of the front-end core or not. If it will be moved to utility, frontend-dynamic-style-sheet can add the org.jetbrains.kotlin-wrappers:kotlin-css dependency in the api mode (so it won't be needed to add any dependencies explicitly to construct styles for dynamic style sheets). Possibly some other front-end libraries can expose their dependencies in the api mode as well.