Get the workspace folder where the task is defined

[P403n1x87]

There doesn't seem to be a way to find out which folder contains the tasks.json file that declared the task that the user wants to run. This makes it impossible to set the current working directory when spawning an external process without making a potentially flawed assumption, like either getting the first entry in vscode.workspace.workspaceFolders or relying on the URI of an open editor (which might not be there).

Unless this feature is implemented, it probably won't be possible to provide a satisfactory fix for issues like P403n1x87/austin-vscode#19.