Make data/ directory automatically accessible to exercises

[rossellhayes]

This adds two interfaces for making files available to learnr tutorials.

1. Any files in a data/ directory in the same directory as the tutorial Rmd document will be accessible to the tutorial.
2. Calling the function use_remote_files() in the setup chunk will cache files into a temporary directory and make them available to the tutorial as if they were in the data/ directory.

[CLAassistant]

All committers have signed the CLA.

[gadenbuie]

This is a great start! I really like how the interface is coming together and I think it will benefit many tutorial authors.

We'll need to think through and account for all of the various ways the underlying options can be set and that files can be added to an exercise. To help with that, I think the next step would be to add several small test tutorials in a folder at tests/testthat/exercise-files. Each one can demonstrate an expected use case. Actually writing the tests is going to be a little tricky, but we can work through that step together. In the mean time it will be helpful to have a few examples to test out interactively.

[gadenbuie]

@rossellhayes I had a few small changes to propose and in the process I merged main into this branch. It turns out that our options-protection work misses options that are NULL. In other words withr::local_options() doesn't reset options that were NULL, so our tests are now failing until we fix that.

options(thing = NULL)
withr::with_options({
  options(thing = "bad")
})
getOption("thing")
#> "bad"

To be fair, this same issue will happen with the standard base R pattern too:

options(thing = NULL)
local({
  opts <- options()
  on.exit(options(opts), add = TRUE)
  options(thing = "bad")
})
getOption("thing")
#> "bad"

We'll have to write a custom options_restore() function that does handle NULL options.

[rossellhayes]

It looks like this is a known issue: r-lib/withr#57
Will Landau proposes a solution here: r-lib/withr#57 (comment)

Edit: fixed in 6b32eeb

[gadenbuie]

This looks great and I'm really excited for this feature to land in learnr. I'm approving the PR but we have a little bit more to do in terms of documentation.

• Add an entry in NEWS under New Features
• Document the new behavior in the learnr docs webpage. I think it makes sense to add a new section in docs/exercises.Rmd after Exercise Setup.
• Once the above are done and the PR is merged, we'll need to follow up in any related issues to let everyone know about the new feature.

[schloerke]

LGTM given cosmetic suggestions!