Add prefer-export-from rule

[fisker]

Fixes #334

[sindresorhus]

With this case:

import foo from 'foo';

unicorn(foo);

export default foo;

I would personally prefer this instead (re-exporting, even if it was imported):

import foo from 'foo';

unicorn(foo);

export default from 'foo';

As it's IMHO more readable and also makes a guarantee that the export is just re-exported and not modified on the way.

Thoughts?

[fisker]

Personally, I only use export...from if it's not used(at least not modified, but we can't static analysis it), imaging this

import foo from 'foo';

foo.bar = 1;

export default from 'foo';

Still prefer export...from ?

[sindresorhus]

Still prefer export...from ?

Yes. It's still more readable. You can immediately see what is exported from where instead of having to read and understand all the code in the file.

[fisker]

These seem strange, I don't have time to confirm how should them work, but I guess we are fine to ignore it for now.

[sindresorhus]

Seems like a bug in ESLint. Both of those should be valid.

[fisker]

This PR has been a while, I forgot if I left something to do, but seems fine to merge.

[sindresorhus]

Thanks for finishing this 👍