Prefer proposal-composites to proposal-richer-keys

It seems [proposal-richer-keys](https://github.com/tc39/proposal-richer-keys) has been unmaintained for many years, whereas [proposal-composites](https://github.com/tc39/proposal-composites) is being actively developed. The latter has a polyfill with tests, which you could bring over quite easily. Would you please consider switching?