feat!: esm, eslint 9, jest 30, vite/vitest opt-in, node 20+

[bjnewman]

Summary

Major modernization of the workflow toolchain.

Changes

• ESM — all packages converted to native ES modules
• ESLint 9 flat config support (requires new eslint-config-availity)
• Jest 27 → 30, jest-environment-jsdom now bundled
• Vite + Vitest available as opt-in alternative (bundler: 'vite')
• Node 20+ required (18 dropped)
• Dropped legacy polyfill support — core-js and IE11 polyfills removed since all supported browsers have these features natively
• Improved error handling — silent catch blocks replaced with diagnostic output
• workflow-upgrade rewritten to use package manager commands and includes ESLint flat config migration

Breaking Changes

• Node 18 no longer supported
• ESLint flat config only (.eslintrc no longer loaded)
• ESM packages — require() will not work
• core-js polyfills no longer injected — if your app still needs polyfills for specific features, add core-js directly

Blocked On

• eslint-config-availity major version publish — version in package.json needs updating after merge

Testing

• Validated against internal applications using both yarn classic and yarn 4

See UPGRADE.md for the full migration guide.

[LauRoxx]

missing 24

[LauRoxx]

Do we want more documentation? We could add the vite config info to the Testing libraries page. On a quick glance through the docs that's the main thing that stuck out

[gregmartDOTin]

Do we want more documentation? We could add the vite config info to the Testing libraries page. On a quick glance through the docs that's the main thing that stuck out

Added some docs.