Add tsdown for cjs and esm builds

[abhithesys]

What

Migrate all packages to dual ESM/CJS builds using tsdown, and add CJS entry points for react-ui. This eliminates the need for transpilePackages in Next.js and build.transpile in Nuxt, fixing the friction reported in #360 where consumers had to manually configure their bundlers to use OpenUI packages.

fixes point 3 in #360

Changes

• Migrated lang-core, react-lang, react-headless, react-email, and openui-cli builds from tsc to tsdown with dual ESM (.mjs) + CJS (.cjs) output and co-located type declarations (.d.mts, .d.cts)
• Added CJS bundles for react-ui (via tsdown) alongside its existing tsc-based ESM build for the . and ./genui-lib entry points
• Updated package.json exports maps across all packages with proper import/require conditional exports
• Added react-headless exports field and "type": "module" (was previously CJS-only with no exports map)
• Added typecheck, check:publint, and check:attw scripts to each package
• Installed tsdown, publint, and @arethetypeswrong/cli as root devDependencies
• Removed transpilePackages from all Next.js example/docs configs and build.transpile from the Nuxt example config
• Updated root and package-level ESLint configs to ignore *.config.ts files

Test Plan

• Verified locally
  • Built all packages with pnpm build and confirmed dual-format output files exist
  • Ran publint and attw checks against built packages
  • Ran existing unit tests (vitest run) for lang-core, react-lang, react-headless
  • Verified Next.js examples resolve packages correctly without transpilePackages