[WIP] Allow for named imports in React.lazy

[gnestor]

• Fix tests
• Update docs: https://github.com/reactjs/reactjs.org/blob/5284091bdd21e393fb1ddbee7ebe98a3ad560356/content/docs/code-splitting.md

Closes #14603

[gaearon]

Thanks for the PR but it's an intentional design decision to not currently allow named imports. We may reconsider it later. You can find the relevant discussion in reactjs/rfcs#64.