Add several basic predefined variables for use in launch.json - e.g. ${uid}, ${gid}, ${user}, ${home}

Please expose several basic predefined variables for use in launch.json like ${uid}, ${gid}, ${user}, ${home} (and potentially in settings.json too, as per https://github.com/microsoft/vscode/issues/2809).

Example use cases:

1. Extremely useful for multi-user environments to avoid port or path conflicts. (See YAML below)
2. Generally similar use cases to https://github.com/microsoft/vscode/issues/2809 where one needs to specify paths to SDKs, GOPATHs and alike.

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "perl",
      "request": "launch",
      "name": "Perl-Debug",
      "console": "remote",
      "port": "${uid}",  // <---- HERE!
      "root": "${workspaceFolder}/.vscode/root",
      "program": "${workspaceFolder}/${relativeFile}",
      "stopOnEntry": false,
      "reloadModules": false,
      "sessions": "watch",
    }
  ]
}

[weinand]

@ssbarnea did you try to use an environment variable that is defined outside of VS Code? You can easily access that as `${env:variable}´.

[weinand]

@ssbarnea I was actually suggesting to introduce a new env variable and assign it to different values on the different platforms.