Export types for npm package

[elsaperelli]

Summary

Had trouble using the fhir-spec-tools as an npm package in bulk-export-server so this PR aims to fix that by hopefully properly exporting the types.

New behavior

Hopefully can use package now.

Code changes

• src/index.ts - specifically export types

Testing guidance

• Question: when I publish this should I bump the version?
• npm publish --dry-run types/index.js and types/index.d.ts should be added.