feat: ability to reference previous llm outputs via storeOutputAs

[typpo]

This makes it easier to build multi-turn conversations. storeOutputAs records the previous LLM output as a variable.

prompts:
  - "Respond to the user: {{message}}"

providers:
  - openai:gpt-4o

tests:
  - vars:
      message: "What's your favorite fruit? You must pick one. Output the name of a fruit only"
    options:
      storeOutputAs: favoriteFruit
  - vars:
      message: "Why do you like {{favoriteFruit}} so much?"
    options:
      storeOutputAs: reason
  - vars:
      message: 'Write a snarky 2 sentence rebuttal to this argument for loving {{favoriteFruit}}: \"{{reason}}\"'