Resolving cjs files issues

`unistore/preact` always resolves to a cjs file
and I think that `unistore/full/preact` resolves to a cjs file too, there are `.es.js` files in this directory, but Im not aware if any tool picks that up automatically over regular `.js` files

The issue can be fixed by introducing "proxy" directories like [this](https://github.com/redux-saga/redux-saga/blob/0d409bd2344032ca09a7c6962c56d28ea0917d44/effects/package.json). Ideally such directories could be outputted by `microbundle`.