feat(contents_text): Add contents_*() helper functions

[gadenbuie]

Fixes #167

Adds contents_text(), contents_markdown() and contents_html() generics that can be applied to Turn and Content objects.

turns <- list(
  user_turn("What's this image?", content_image_url("https://placehold.co/200x200")),
  Turn("assistant", "It's a placeholder image.")
)

lapply(turns, contents_text)
#> [[1]]
#> [1] "What's this image?"
#> 
#> [[2]]
#> [1] "It's a placeholder image."
lapply(turns, contents_markdown)
#> [[1]]
#> [1] "What's this image?\n\n![](https://placehold.co/200x200)"
#> 
#> [[2]]
#> [1] "It's a placeholder image."
contents_html(turns[[1]])
#> [1] "<p>What's this image?</p>\n\n<img src=\"https://placehold.co/200x200\">"

[hadley]

This seems like a reasonable idea and implementation to me. You just need to fix the build failure and add some basic tests.

[hadley]

Can you pass turns?

[gadenbuie]

I just added a method for contents_markdown() for Chat instances. I see a lot of utility in that particular output format and less utility in the HTML or text variants. That said I'd be happy to add those if you want.

turns <- list(
  user_turn("What's this image?", content_image_url("https://placehold.co/200x200")),
  Turn("assistant", "It's a placeholder image.")
)

chat <- Chat$new(Provider("https://example.com"), turns = turns)
cat(contents_markdown(chat))
#> ## User
#> 
#> What's this image?
#> 
#> ![](https://placehold.co/200x200)
#> 
#> ## Assistant
#> 
#> It's a placeholder image.

[gadenbuie]

Related: is there a way to create a chat_echo() (or maybe chat_repeater()) chat instance that just returns the user input and doesn't call any APIs? Carson made an example like this for Shiny and I've found it to be very useful for debugging and examples when I'm not ready for real model usage yet.

[hadley]

I think that would be pretty straightforward with a new provider.

[gadenbuie]

@hadley this is ready for another look. Sorry for the commit noise, I only just now realized I wasn't able to run checks locally because I had out-of-date credentials. Of course when I finally got to ✅ in the last commit there was an unrelated CI failure and I can't request a rerun.

[cpsievert]

I love this idea in general, but also had a couple thoughts on this PR:

1. It'd be nice if these content extractors had the ability to get "additional" content (i.e., the stuff you get with $chat(echo="all")), mainly for debugging (but also educational) purposes.
2. It feels like the main way people would want to use this is to generate an md/html file of the whole conversation? For that reason, instead of generics, I think for chatlas I might do something like chat.export(filename="output.html", content="all"), which might internally call a new turn.get_content("all").