Add a 'viewsWelcome' to suggest creating a new F# project

[baronfel]

@Happypig375 wondered why we couldn't suggest creating an F# project on the following view:

This turns out to be pluggable in vscode, via the contributes.viewsWelcome extension point. We should consider adding a suggestion here, something of the form:

{
  "contributes": {
    "viewsWelcome": [
      {
        "view": "explorer",
        "contents": "You can also open a folder containing an F# solution, or create a new F# project by clicking the button below.\n[Create F# Project](command:fsharp.NewProject)",
        "when": "workbenchState == empty" // TBD the appropriate condition here
      }
    ]
  }
}

This should be pretty simple since we already have the 'new project' command, we just need to wire it up.

[baronfel]

uh, it's actually hard. we need a 'rootpath' to invoke the dotnet cli in, and programmatically setting that is more difficult than I imagined. even then, if we do set it, we probably need to re-trigger the initialization routines after the project is added, because it would mean the creation of a new workspace.