Publish native ESM alongside CommonJS

[robgruen]

Summary

Closes #120.

• Publish native ESM JavaScript and declarations in dist/esm alongside the existing CommonJS output in dist.
• Add conditional import/require exports and format-specific declarations for typechat, typechat/ts, typechat/zod, and typechat/interactive.
• Use explicit .js relative imports and compiler imports that work in both module formats.
• Build the library before workspace examples, document both consumption styles, and run the test suite in Node.js CI.

Compatibility

• Preserve existing CommonJS output paths and public APIs.
• Keep the ES2021 JavaScript target and existing Node.js >=22.19.0 requirement unchanged.
• Add no dependencies.

Validation

Validated on Node.js 22.19.0:

• npm test: 145 tests passed, including new package compatibility checks.
• Clean dependency restoration and npm run build-all: all workspace examples built.
• Actual npm tarball: all four entry points loaded with both native import and CommonJS require.
• ESM/CommonJS named export parity and behavior checks for translation, TypeScript/Zod validation, programs, and interactive input.
• Format-specific declaration resolution under both node16 and nodenext, including negative type checks.
• Core and interactive entry points load without optional peer dependencies.