Fix transpilation of for of loops with Set caused by dangerousForOf in Buble config.

[parkerziegler]

This PR fixes a major 🐛 in v0.6.0. Previously, we had dangerousForOf enabled in our Buble transforms. dangerousForOf assumes all structures being iterated with for...of loop are array-like objects with a length property 😱 . This doesn't work for us because we iterate over a Set using for...of.

This PR adds in the necessary Babel transform to support IE11: https://babeljs.io/docs/en/babel-plugin-transform-for-of For now I'd like to keep IE11 support, but it is clear from the example that we end up adding a pretty hefty transpilation cost with this plugin. In a future release of renature (likely v1), we'll remove IE11 support entirely and strip this plugin. But for now, it makes sense to keep it in.

[kitten]

In urql we'd typically not use for...of but would hand write for-each loops instead. Given that you're about to pull in a lot of helper code potentially for the iterables, is that maybe a preferable option?

[parkerziegler]

Good shout. The more I think about it too, the more I'm less inclined to include bundle bloating code for IE11. All modern browsers natively support for...of so I actually think I just won't include any transforms for it ✅