Executable prompts implementation is a mess and needs refactoring

[pavelgj]

I know because I implemented it... :P

There are inconsistencies between underlying definePrompt and defineDotprompt implementations and some features are partially supported under the ai.definePrompt umbrella. Some inconsistencies are worked around with hacks where it requires a deeper refactoring.

[MichaelDoyle]

Can I also make an appeal to have separate trace spans for render and generate? :) We started down this path for dotprompt, and it was quite nice!