The existing ESM entry points (`libphonenumber-js`, `/min`, `/max`,
`/mobile`, `/core`) are named "es6" internally, but they aren't actually
modern-syntax builds: `@babel/preset-env` runs without a target and therefore
still transpiles classes, destructuring, `for...of`, etc. to older JavaScript.
That keeps backwards compatibility, but modern consumers pay for Babel helper
code that their own browser targets don't need.

Keep those existing entry points unchanged and add opt-in modern subpackages
instead:

* `libphonenumber-js/min/es6`
* `libphonenumber-js/max/es6`
* `libphonenumber-js/mobile/es6`
* `libphonenumber-js/core/es6`

The default package entry points remain on the current transpilation path, so
this is not a breaking change. Consumers that want the smaller modern build can
opt in explicitly by importing from the new `/es6` subpaths.

The new `BABEL_ENV=es6-modern` build writes source modules to `es6-modern/`
using `@babel/preset-env` with `modules: false`, `bugfixes: ...

Add modern ESM subpackages

See merge request !29

The existing ESM entry points (`libphonenumber-js`, `/min`, `/max`,
`/mobile`, `/core`) are named "es6" internally, but they aren't actually
modern-syntax builds: `@babel/preset-env` runs without a target and therefore
still transpiles classes, destructuring, `for...of`, etc. to older JavaScript.
That keeps backwards compatibility, but modern consumers pay for Babel helper
code that their own browser targets don't need.

Keep those existing entry points unchanged and add opt-in modern subpackages
instead:

* `libphonenumber-js/min/es6`
* `libphonenumber-js/max/es6`
* `libphonenumber-js/mobile/es6`
* `libphonenumber-js/core/es6`

The default package entry points remain on the current transpilation path, so
this is not a breaking change. Consumers that want the smaller modern build can
opt in explicitly by importing from the new `/es6` subpaths.

The new `BABEL_ENV=es6-modern` ...