Provide R package-related tasks

There are some things added in one of the recent releases that we should consider including as VS Code tasks as well.

In particular, being able to run tests and R CMD check is ideally-suited to tasks.

I wasn't sure whether an extension can provide tasks in this way, but it appears to be possible. I don't know TypeScript well enough to implement, but here's an example of an extension that provides users with tasks: https://github.com/Microsoft/vscode-extension-samples/tree/master/task-provider-sample

I can help in terms of putting together the task definition JSONs since I'm using some in my own projects already.