Support .mjs plugins/presets and async factories

[nicolo-ribaudo]

Q	A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

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.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/32270/

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Latest deployment of this branch, based on commit 52e0f20:

Sandbox	Source
babel-repl-custom-plugin	Configuration
babel-plugin-multi-config	Configuration

[JLHwung]

nit: By default env is passed with process.env. https://nodejs.org/api/child_process.html#child_process_child_process_execfile_file_args_options_callback

[nicolo-ribaudo]

Actually, I need someone with macOS to check if I need to do https://github.com/babel/babel/blame/31396b286daadee3c25c4891c8dd31a92d7f0604/packages/babel-core/test/config-chain.js#L63 for this test.

[JLHwung]

make prepublish is passing as of dfa75fa on macOS.

[nicolo-ribaudo]

I'm merging this PR to the feat-7.13.0/babel-core-features branch, because there are three different PRs that modify @babel/core that conflict with each other (this one, assumptions and targets)