Fix ESM package entrypoints

[jahusa02]

Summary

• route package-root import to the existing ESM build and require to an explicit CommonJS entry via an exports map
• point main to build/ibantools.cjs and generate that file during the node build so type: module no longer breaks CJS resolution
• add a package-root regression test covering both named ESM imports and CommonJS require('ibantools')

Verification

• npm test
• npm run lint

Closes #672

[coveralls]

coverage: 88.921% (-10.8%) from 99.766% — jahusa02:fix/672-esm-entrypoints into Simplify:master

[Simplify]

@jahusa02 thank you for your contribution!