Allow projects to suggest extensions

[vcsjones]

It would nice if a project had a way to advertise what extensions are required or helpful within the context of a project. Perhaps a new file in the .vscode directory that allowed listing extensions and versions. For example:

{
    "typescript": "~1.8.0",
    "tslint": "~0.5.32"
}

This will help people that are just getting started on a project understand what dependencies are required to use the project.

When opening the project, VS Code might say something like, "This project has recommended extensions. Would you like to review them?"

[mdijoux]

I think that this is the same as if we had a thing like #347

[Telokis]

Isn't this issue closed by https://code.visualstudio.com/docs/editor/extension-gallery#_workspace-recommended-extensions?

[mjbvz]

Thanks @Telokis! Yes I think workspace recommended extensions covers this