Add document key for repos

[coatless]

With the arrival of reproducible actions at r-wasm/actions, package authors can now set up a CRAN-like repository with an R WASM binary and deploy it to the repository's GitHub Pages. As a result, we likely want to support setting the repos option inside of the document yaml.

webr:
   repos: ["https://to.github.io/webr/repo"]

[coatless]

We sketched out a proof of concept related to how to obtain development binaries over yonder

https://github.com/coatless-tutorials/webr-github-action-wasm-binaries

Maybe add a section related to usage in the using-packages section.

[coatless]

Psst @jimjam-slam check out the repos key ;)

https://quarto-webr.thecoatlessprofessor.com/tests/qwebr-test-repo-key.html
https://github.com/coatless/quarto-webr/blob/main/tests/qwebr-test-repo-key.qmd

You might also like:
https://github.com/coatless-tutorials/webr-unified-gh-workflow

[jimjam-slam]

Ooooooh this is cool!