Allow css tag to be exported

[dndrsn]

Describe the enhancement

Allow/enable export of the css tag from another file. For example...

// ./style.js

export { css } from '@linaria/core';

...

// ./other-file.js

import { css, somethingElse, anotherExport } from './style.js';

When this is done currently, the following error is produced...

Error: Using the "css" tag in runtime is not supported. Make sure you have set up the Babel plugin correctly.

Motivation

This would be convenient for organizational flexibility. In my use case, I have helper functions that are naturally imported with the css tag. I would prefer to import from a single file with a single line.