feat: prompt mixin (#543)

apps/docs/docs/modules/prompt/_category_.yml

apps/docs/docs/modules/prompt/index.md

examples/prompts/promptMixin.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/engines/chat/CondenseQuestionChatEngine.ts

packages/core/src/engines/chat/ContextChatEngine.ts

packages/core/src/engines/chat/DefaultContextGenerator.ts

packages/core/src/engines/query/RetrieverQueryEngine.ts

packages/core/src/engines/query/RouterQueryEngine.ts

packages/core/src/engines/query/SubQuestionQueryEngine.ts

packages/core/src/index.ts

packages/core/src/indices/vectorStore/VectorStoreIndex.ts

packages/core/src/prompts/Mixin.ts

packages/core/src/prompts/index.ts

packages/core/src/selectors/base.ts

packages/core/src/selectors/llmSelectors.ts

...es/core/src/synthesizers/MultiModalResponseSynthesizer.ts

packages/core/src/synthesizers/ResponseSynthesizer.ts

packages/core/src/synthesizers/builders.ts

packages/core/src/synthesizers/types.ts