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: true`, `loose: true`
and `targets: { esmodules: true }`. That lets Babel choose the native-ES-module
browser cutoff from its own compat data instead of maintaining a hand-picked
browser version list in this package.

The `/es6` subpackage entry files follow the existing package layout: `core/es6`
points at the modern Babel output, while `/min/es6`, `/max/es6` and
`/mobile/es6` mirror their current barrels and export wrappers with imports
redirected through `core/es6`. Only the class wrappers are adjusted to use
native `extends`/`super`, because the modern core build emits native classes,
while preserving `PhoneNumber` `instanceof` compatibility for parsed phone
numbers.

The existing `BABEL_ENV=es6` build is preserved as the status quo. Its output
remains byte-for-byte identical to the previous config; `transform-for-of` is
now scoped to the legacy ESM/CommonJS/coverage build environments rather than
being applied globally.

Measured with a consumer bundle using Rollup + Terser (`ecma: 2020`) and a
public-API import set:

                        | legacy raw | modern raw | legacy gzip | modern gzip
  min                   | 177,666 B  | 140,563 B  | 42,081 B    | 38,223 B
  max                   | 251,327 B  | 214,224 B  | 62,690 B    | 58,750 B
  mobile                | 193,117 B  | 156,014 B  | 46,712 B    | 42,682 B
  core                  |  93,076 B  |  56,158 B  | 22,308 B    | 18,223 B

That is roughly 37 KB raw and 3.8-4.1 KB gzip saved, while preserving the
current default behavior for existing users.

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` build writes source modules to `es6-modern/`
using `@babel/preset-env` with `modules: false`, `bugfixes: true`, `loose: true`
and `targets: { esmodules: true }`. That lets Babel choose the native-ES-module
browser cutoff from its own compat data instead of maintaining a hand-picked
browser version list in this package.

The `/es6` subpackage entry files follow the existing package layout: `core/es6`
points at the modern Babel output, while `/min/es6`, `/max/es6` and
`/mobile/es6` mirror their current barrels and export wrappers with imports
redirected through `core/es6`. Only the class wrappers are adjusted to use
native `extends`/`super`, because the modern core build emits native classes,
while preserving `PhoneNumber` `instanceof` compatibility for parsed phone
numbers.

The existing `BABEL_ENV=es6` build is preserved as the status quo. Its output
remains byte-for-byte identical to the previous config; `transform-for-of` is
now scoped to the legacy ESM/CommonJS/coverage build environments rather than
being applied globally.

Measured with a consumer bundle using Rollup + Terser (`ecma: 2020`) and a
public-API import set:

                        | legacy raw | modern raw | legacy gzip | modern gzip
  min                   | 177,666 B  | 140,563 B  | 42,081 B    | 38,223 B
  max                   | 251,327 B  | 214,224 B  | 62,690 B    | 58,750 B
  mobile                | 193,117 B  | 156,014 B  | 46,712 B    | 42,682 B
  core                  |  93,076 B  |  56,158 B  | 22,308 B    | 18,223 B

That is roughly 37 KB raw and 3.8-4.1 KB gzip saved, while preserving the
current default behavior for existing users.