Add support for "default" export behavior in ECMAScript Modules

[nurdism]

Using babble to write my plugins, running into an issue with export default foo. Simple fix/solution would be to add :

  if (typeof fn.default !== "undefined" ) {
    fn = fn.default
  }

before https://github.com/fastify/fastify-plugin/blob/master/index.js#L8

[mcollina]

Would you like to send a PR to add this?
Would this just support transpiled modules, or actual node syntax with .mjs?

[nurdism]

this is just for transpiled modules from babel, and yeah I can make a quick PR for it