feat: use conditional exports (#401)

.changeset/popular-grapes-cheat.md

.prettierignore

examples/chatEngine.ts

examples/chatHistory.ts

examples/keywordIndex.ts

examples/sentenceWindow.ts

examples/storageContext.ts

examples/subquestion.ts

examples/summaryIndex.ts

examples/tsconfig.json

examples/vectorIndexCustomize.ts

packages/core/package.json

pnpm-lock.yaml

tsconfig.json