Prepare the `rmarkdown::render()` code snippet for the challenge of PowerShell

[jennybc]

WIP, still not working on all OSes. I'm growing increasingly pessimistic that these code snippets can be written in a way that works on all OSes, leaving the quoting to VS Code.

Addresses #3816

QA Notes

Exercise these commands on multiple OSes, from the command pallette:

• R: Render Document with R Markdown
• R: Source R File