add callback and event tracking

.gitignore

apps/simple/chatEngine.ts

apps/simple/listIndex.ts

apps/simple/openai.ts

apps/simple/simple.txt

apps/simple/subquestion.ts

apps/simple/vectorIndex.ts

packages/core/package.json

packages/core/src/BaseIndex.ts

packages/core/src/ChatEngine.ts

packages/core/src/Embedding.ts

packages/core/src/GlobalsHelper.ts

packages/core/src/LLMPredictor.ts

packages/core/src/LanguageModel.ts

packages/core/src/NodeParser.ts

packages/core/src/OutputParser.ts

packages/core/src/Prompt.ts

packages/core/src/QueryEngine.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/Response.ts