fix: resolve ESM import failure in v4.0.1 (fixes #158)

- Fix Node.js ESM module resolution by creating proper dual package structure
- Add package.json files in dist/esm/ and dist/cjs/ to override module types
- Create postbuild script to add .js extensions to relative imports in ESM files
- Update build scripts to use cross-platform commands (npx instead of ./node_modules/.bin/)
- Add cross-env dependency for Windows compatibility in test scripts
- Bump version to 4.0.2

Fixes #158 - ESM imports now work correctly in projects with type: module.
Both CommonJS and ESM imports are fully supported without breaking changes.

Author: hnviradiya