Add option to pre-download files

[terhorst]

Thanks for the really useful extension!

Pulling data into the web console is rather difficult -- many websites do not supply the necessary CORS headers, which causes download.files() to fail. (This is a known issue with webR.)

It would be helpful to add a files: [] clause to the configuration. You could map over this list similar to what is already done with packages:

quarto-drop/drop-runtime/src/App.tsx

Line 83 in 4a4b3cf

await Promise.all(packages.map((pkg) => webR.installPackages(pkg)));

The necessary webR call would be something like webR.evalRvoid("download.files(...)"). But I'm not really sure where the file would end up/I don't understand the webR virtual filesystem in general. The desired behavior would be that specifying e.g.

format:
  revealjs:
    drop:
      engine: webr
      webr:
        files:
          - https://a.url/data.csv

would enable me to run read_csv("data.csv") later on and have it just work.