feat(react): add react agent (#511)

apps/docs/docs/modules/agent/openai.mdx

apps/docs/docs/modules/agent/react_agent.mdx

examples/agent/react_agent.ts

packages/core/src/agent/index.ts

packages/core/src/agent/openai/worker.ts

packages/core/src/agent/react/base.ts

packages/core/src/agent/react/formatter.ts

packages/core/src/agent/react/outputParser.ts

packages/core/src/agent/react/prompts.ts

packages/core/src/agent/react/types.ts

packages/core/src/agent/react/worker.ts

packages/core/src/llm/LLM.ts

pnpm-lock.yaml