feat: support es modules without .mjs extension

[snyamathi]

This PR consolidates the import and require logic into a single function, loadModule which can load a config using import for node 12+ and require otherwise for legacy Node.

An import statement can reference an ES module or a CommonJS module. import statements are permitted only in ES modules, but dynamic import() expressions are supported in CommonJS for loading ES modules.
https://nodejs.org/api/esm.html#interoperability-with-commonjs

When importing a default export with dynamic imports, it works a bit differently. You need to destructure and rename the "default" key from the returned object.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#importing_defaults

This has a consequence with modules which are transpiled from ESM to CJS, often using something like babel-plugin-transform-modules-commonjs. This effectively adds a second layer of .default that we may need to dereference.

// src
export default [/* config */];

// dist
Object.defineProperty(exports, "__esModule", {
  value: true,
});

exports.default = [/* config */];

Let me do some final testing though - and let's not merge this on a Friday cause I'm done for the week 😄

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[snyamathi]

This file and the one above it are intentionally formatted to look nearly identical. The only real difference between the two is that when doing a dynamic import we have to dereference the default export. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#importing_defaults

[snyamathi]

https://github.com/yahoo/ycb-config/pull/34/files?w=1