Add TypeScript/ESM build pipeline while keeping legacy bundle intact

[s-meiser]

Summary

• add new TypeScript sources under src-ts/ and emit ESM, CJS, and .d.ts outputs
• wire npm scripts and tsconfig files for the modern builds plus a lightweight terser-based UMD bundle
• retain the original Gulp pipeline and dist/accordion.min.js so existing consumers remain unaffected
• refresh project docs to explain the new build flow and the optional legacy regeneration step

Motivation

• Tree-shaking through the ESM build lets modern bundlers include only what is used, reducing payload size
• CJS + types give Node/TypeScript users first-class support without external wrappers
• Zero migration for direct <script> / CDN users because the legacy UMD bundle and Gulp workflow stay intact

Build Outputs

• ESM: esm/
• CJS: cjs/
• Types: dist-types/
• UMD (legacy + terser rebuild): dist/accordion.min.js

package.json exposes the builds like this:

{
  "main": "dist/accordion.min.js",
  "module": "esm/index.js",
  "types": "dist-types/index.d.ts",
  "exports": {
    ".": {
      "types": "./dist-types/index.d.ts",
      "import": "./esm/index.js",
      "require": "./cjs/index.js",
      "default": "./dist/accordion.min.js"
    },
    "./dist/accordion.min.css": "./dist/accordion.min.css"
  }
}

Testing / Verification

1. Build modern outputs: npm run build
2. Regenerate the legacy bundle: npm run build:legacy
3. (Optional) produce the terser UMD bundle after building TS outputs: npm run build:bundle
4. Manual smoke tests (optional):
   • node -e "import('./esm/index.js').then(m => console.log(Object.keys(m)))"
   • node -e "console.log(Object.keys(require('./cjs/index.js')))"

Size (legacy UMD, minified)

before	after	delta
7 144 B	7 110 B	−34 B (~0.48%)

Docs

• Updated build instructions to cover modern outputs, legacy regeneration, and optional UMD rebuild

Backward Compatibility

• No breaking changes expected: the legacy UMD bundle remains the default export path
• Modern bundlers can opt into the ESM/CJS builds transparently through exports

Notes / Follow-ups

• Consider documenting minimum supported Node / TypeScript versions in the README
• Future CI could run npm run build, npm run build:legacy, and a minimal import smoke test for both module formats