Mellea should warn about mega-prompt anti-patterns / code smells

[nrfulton]

Users have a tendency to dump very large prompts into instruct or genslot. This bad for two reasons. First, it's not melleaic. Second, when doing this, m.chat should be used.

We should add a warning/linter that captures these behaviors and warns the user.

Some suggestions from David:

For Instruction and GenerativeSlot in particular, Mellea should check the incoming arguments to see if they are in the right spirit and warn if they are not (possibly only if a "warning" flag is set. Same for generative stubs (and even more pronounced): does this look like a docstring, or did someone just put an agent prompt in a docstring?

[s1v4-d]

Could I work on this? @nrfulton

[s1v4-d]

#272 @nrfulton