Add Node ESM loader hook for import styles from './x.css'

[madyankin]

Summary

• Adds an opt-in postcss-modules/loader entrypoint — a Node module-customization hook that lets JavaScript files do import styles from './foo.css' and receive the token map directly. Mirrors the workflow css-loader and Vite's built-in CSS-Modules handling already provide.
• Aimed at non-bundler use cases (SSR, Node scripts, Jest/Vitest in node mode). Bundler users keep using their existing integrations; nothing about the existing plugin path changes.
• Options resolved from postcss-modules.config.{js,cjs,mjs} in cwd, POSTCSS_MODULES_CONFIG env var, or a "postcss-modules" key in package.json. Same shape as the PostCSS plugin options.
• Closes Deeper integration with postcss-loader #80.

Usage

node --import postcss-modules/loader app.mjs

// app.mjs
import styles from "./button.css";
console.log(styles.primary); // "_button__primary_xkpkl_5"

Implementation notes

• src/loader.mjs — entry; calls module.register on the hooks module.
• src/loaderHooks.mjs — load hook intercepts file://*.css URLs, runs them through the existing plugin with a getJSON callback that captures tokens, returns a synthetic ESM source.
• src/processCSS.js — small helper that sets up the fs adapter and invokes the existing makePlugin pipeline (no new code path through the transformer).
• src/loadConfig.mjs — config resolver. Kept as .mjs so swc doesn't rewrite its dynamic import() to require().
• Makefile gets a compile target; swc ignores *.mjs and we cp those verbatim into build/.
• package.json exports map adds "./loader". engines.node bumped to >=20.6 (where module.register stabilized).

Limitations (called out in the README)

• Returns the token map only; the transformed CSS is not emitted. SSR users who also need the CSS should use a bundler.
• No HMR / watch mode — by design.

Test plan

• make test — 42 passing (5 new loader cases + 37 existing), lint clean.
• New cases cover: simple class set, composes-across-files, postcss-modules.config.cjs discovery from cwd, POSTCSS_MODULES_CONFIG env var override, missing-file error surfaces the source path.
• Smoke test against a real consumer:

  mkdir /tmp/pcm && cd /tmp/pcm && npm init -y && npm link postcss-modules
  echo '.primary { color: red; }' > button.css
  echo 'import s from "./button.css"; console.log(s);' > app.mjs
  node --import postcss-modules/loader app.mjs