This is the first step needed to migrate to native ECMAScript modules: we must support loading .mjs plugins and presets 😛

• The first commit is mostly needed for third-party presets: many of them directly use require() inside the preset factory, and this makes it possible to use await import() instead.
• The second commit enables loading .mjs plugins/presets. It's mostly functions being converted to generators to work with gensync.

While preparing this PR, I noticed that we currently support this kind of presets:

exports.__esModule = true;
exports.plugins = [foo];

which is the result of transpiling

export const plugins = [foo];

However, we throw an error for

exports.__esModule = true
exports.default = { plugins: [foo] }

because presets must export a function, and not a plain object:

exports.__esModule = true
exports.default = function () {
  return { plugins: [foo] };
};

Since presets must export a function (so that they can properly configure caching), I didn't implement support for the named exports in .mjs files.