Utility for vscode build task

[anthonynorth]

Can we add a utility for configuring a vscode build task to run targets::tar_make(). Something like:

tflow::use_vscode(default=TRUE)

Which writes the following to ./.vscode/tasks.json

{
  "version": "2.0.0",
  "tasks": [
    {
      "type": "process",
      "command": "Rscript",
      "args": ["-e", "targets::tar_make()"],
      "label": "tflow: make",
      "group": {
        "kind": "build",
        "isDefault": true
      }
    }
  ]
}