Open User Tasks... command does not work anymore

It's a regression after switching to new fs api.

From logs:
```
/21.bundle.js:932 Uncaught (in promise) Error: 'user_storage:/user/tasks.json' is invalid
    at MonacoEditorProvider.<anonymous> (/21.bundle.js:932)
    at step (/21.bundle.js:800)
    at Object.next (/21.bundle.js:781)
    at fulfilled (/21.bundle.js:772)
```