Cannot run code when opening just a single file

_Originally reported by @LeoZieger_

### Repro

Open a file `test.effekt` in a new folder, never before seen by VSCode:
```sh
FOLDER=$(mktemp -d); cd $FOLDER; code test.effekt
```

Then put the following into the file:
```effekt
def main() = println("Hello world")
```

and click the `|> Run` button above the main.

The output of the above is an error:

> Variable `workspaceFolder` can not be resolved. Please open a folder.

### Context

Everything works fine if you open the enclosing folder, only then create the file in VSCode.

We do seem to use `workspaceFolders` (plural) in a number of places: https://github.com/search?q=repo%3Aeffekt-lang%2Feffekt-vscode%20workspaceFolder&type=code