Create a one-line command to install VS environment

[jonthysell]

As part of PR #4512, I'm updating the Install-VSFeatures.ps1 to much more powerful in terms of installing VS components.

With the right list of components, there's no reason we couldn't create a "new dev environment" setup script, that calls Install-VSFeatures.ps1 and installs everything necessary to build RNW.

Even if it can't "set it up from scratch", if the user already has some part of VS 2019 installed, the script would fill in the gaps.

[asklar]

@chrisglein @jonthysell this is covered by #4335

[asklar]

@jonthysell can we close this now or is there more to do?