Add workflows (#1188)

.changeset/eight-countries-eat.md

apps/docs/docs/modules/workflows.mdx

examples/openai_o1.ts

examples/workflow/README.md

examples/workflow/app-creator.ts

examples/workflow/collect-events.ts

examples/workflow/joke.ts

examples/workflow/stream-events.ts

examples/workflow/timeout.ts

examples/workflow/validation.ts

packages/core/package.json

packages/core/src/workflow/context.ts

packages/core/src/workflow/events.ts

packages/core/src/workflow/index.ts

packages/core/src/workflow/workflow.ts

packages/core/tests/workflow.test.ts