Support R package tasks for packages not at the workspace root

[juliasilge]

Sometimes folks have R packages they are working on that are not at the workspace root, but our support for R packages (tasks, commands, keybindings, Test Explorer) all work only (?) when the package is at the root. Related to #6560.

If folks find themselves in this situation, some workarounds for now include:

• running the relevant code (such as devtools::test()) from the interactive console, after setting the working directory
• opening an additional workspace at the R package root, and doing R package tests there