Assist with Quarto vignettes and articles

[jennybc]

Closes #1997

[jennybc]

@hadley @cderv I'm going to tackle use_article() now and I need to exercise this a bit more. done now

But I need to release usethis in the next week, so I'd like to invite feedback based on what you see here now. You should be able to get the general idea.

@hadley I know we talked about the interface a while back. I hope I've done what we discussed, i.e. inferring Rmd vs. qmd from the name.

@cderv I'm especially interested in you looking at whether I've got the DESCRIPTION, gitignores, and Rbuildignores rigged the way they should be.

Thanks in advance!

[jennybc]

Suffering from this beyond-my-control problem on macOS GHA:

Error: Failed to get R release: Failed to get R 4.4.2: Failed to install qpdf: Error: The process '/opt/homebrew/bin/brew' failed with exit code 1

[jennybc]

This will gitignore .quarto at the top-level (relevant to README.qmd, although usethis isn't helping to place such a file just yet), vignettes/.quarto (normal vignettes), and vignettes/articles/.quarto (articles). It's probably redundant with something Quarto itself is doing, but I think this is better than what Quarto does and redundancy is not a big deal in this context.