Introduce `DebugSession#workspaceFolder`

[tsmaeder]

Feature Description:

Introduce DebugSession#workspaceFolder
See https://code.visualstudio.com/api/references/vscode-api#DebugSession

The workspace folder should be taken into account when running pre-launch/debug tasks, for example, but also in labels where it makes sense. Digging through the source code VS Code might be necessary to identify usages.

[jfaltermeier]

I will start having a look at this issue