Allow us to work on the code as it's going to run
instead of the hybrid ConnonJS-ESM format that cannot run anywhere
natively.
`babel-plugin-add-module-exports` always overwrote module.exports,
so the module.exports.default that Babel created was not
visible to consumers.
I've tested importing the code from CommonJS and native ESM.

* chore: use same quotes everywhere
Use single quotes as in cssnano.
Previously the code used a mix of single and double quotes.

* chore: reformat with prettier

There might be an issue with the latest PostCSS 4.6 which
causes some comments not two be removed any more, so it's best
to investigate that separately.