Add prompt optimizer

[filip-michalsky]

We should explore a prompt optimizer to help users succeed if they prompt somewhat incorrectly.

This is a general idea on stagehand functionality. It may cause it to be more opinionated but users may see more success if the LLM chooses better actions when the users prompt has been improved.

Could even be a flag optimize_prompt=True

example: https://github.com/stanfordnlp/dspy