Provide an esm build for `@next/env`

Link to the code that reproduces this issue

https://github.com/AhmedBaset/next-env-repro-esm

To Reproduce

1. Run node script.js
2. Run node script.mjs

Current vs. Expected behavior

@next/env doesn't work correctly with esm.

This cjs works as expected

require("@next/env").loadEnvConfig(process.cwd());
console.log(process.env.TEST);

But this doesn't

import { loadEnvConfig } from "@next/env";
loadEnvConfig(process.cwd());

console.log(process.env.TEST);

It throws:

import { loadEnvConfig } from "@next/env";
         ^^^^^^^^^^^^^
SyntaxError: Named export 'loadEnvConfig' not found. The requested module '@next/env' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from '@next/env';
const { loadEnvConfig } = pkg;

Let's try both cjs and esm in REPL

Welcome to Node.js v20.12.2.
Type ".help" for more information.

> require("@next/env")
Object [Module] {
  initialEnv: [Getter],
  updateInitialEnv: [Getter],
  processEnv: [Getter],
  resetEnv: [Getter],
  loadEnvConfig: [Getter]
}

> require("@next/env").default
undefined

> await import("@next/env")
[Module: null prototype] {
  default: Object [Module] {
    initialEnv: [Getter],
    updateInitialEnv: [Getter],
    processEnv: [Getter],
    resetEnv: [Getter],
    loadEnvConfig: [Getter]
  }
}

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Home Single Language
  Available memory (MB): 15774
  Available CPU cores: 12
Binaries:
  Node: 20.12.2
  npm: N/A
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 14.2.4 // There is a newer version (14.2.5) available, upgrade recommended!
  eslint-config-next: N/A
  react: 18.3.0-canary-14898b6a9-20240318
  react-dom: 18.3.0-canary-14898b6a9-20240318
  typescript: 5.4.5
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

@next/env

Which stage(s) are affected? (Select all that apply)

ESM modules

Additional context

No response

As a stopgap workaround, we use esm/cjs conditionally

import nextEnv from "@next/env";
import type { Config } from "drizzle-kit";

if (nextEnv && "loadEnvConfig" in nextEnv) {
  nextEnv.loadEnvConfig(process.cwd());
} else {
  // eslint-disable-next-line
  require("@next/env").loadEnvConfig(process.cwd());
}

Start with the reproduction repository at https://github.com/AhmedBaset/next-env-repro-esm and run node script.js and node script.mjs to compare CommonJS and ESM behavior. Investigate the @next/env package entry points and build configuration; done means the named ESM import of loadEnvConfig works while the existing CommonJS usage continues to work.