Feature: require callback per node.js DEP0013

[coreyfarrell]

DEP0013: fs asynchronous function without callback has been a runtime deprecation since node.js 7, an error since node.js 10. I do not think graceful-fs needs to wait for node.js 10 to be the minimum version before it starts imposing this requirement.

Obviously this would be a semver-major change. Are you open to making this change? Do you think callback based functions which are not currently patched should be patched to require a callback (effectively backport the node.js 10 error)?