Reevaluate exports: 'auto'

It's just a request to evaluate if exports: 'auto' is still today the best solution. I understand its nice to use `require('lib')` instead of `require('lib').default`, but imho the BIG problem with having this auto mode **by default** is that whenever a user adds a named export from his/her library it is automatically a **breaking change** and one that might easily slip released as patch/minor because its really simple to miss that.

I'll make piece with either decision, just wanted to raise a concern about this.

I've filled this issue at rollup - https://github.com/rollup/rollup/issues/1881 , but whatever they decide does not have to be the same for microbundle, hence "duplicated" issues.