strong types for prompts

.changeset/fair-pets-leave.md

apps/simple/csv.ts

apps/simple/openai.ts

apps/simple/vectorIndexCustomize.ts

examples/csv.ts

examples/openai.ts

examples/vectorIndexCustomize.ts

packages/core/src/ChatEngine.ts

packages/core/src/Prompt.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/ResponseSynthesizer.ts

packages/core/src/indices/summary/SummaryIndexRetriever.ts