Use native ESM for dev scripts

Since we are now only building Babel on Node.js 14 (even if we still test it on Node.js 6), we can use native ECMAScript modules for the build scripts:

• Top-level scripts There are a bunch of scripts/tests in the scripts folder: they can probably all be rewritten to ES modules. We can probably add "type": "module" to the top-level package.json, renaming config files (such as .eslintrc.js) to .cjs. Note that we cannot use ES modules for babel.config.js and jest.config.js, because those two files are used to run tests on Node.js 6.

• babel-types/scripts can be rewritten to ES modules. These files should be renamed to .mjs, to keep the dist @babel/types files with the .js extension (they are CJS modules).

Please note that babel-types/scripts depends on the top-level scripts, so babel-types/scripts must be converted first (it cannot synchronously import an ESM file while still being CJS).

In order to test if your changes are still ok, running yarn (only when you clone the repo) and make build && yarn jest should be enough.

[JLHwung]

Note that we should also bump building requirements to ^12.16 || >= 14 in https://github.com/babel/babel/blob/main/CONTRIBUTING.md#developing

We can also update gulp to 4.2.0 which supports .mjs config.

[karansapolia]

@nicolo-ribaudo Yes, I was stuck on an error. Could use some help definitely. Let me create a draft PR so all can have a look.