There are new settings to limit the maximum number of editors that are open at
the one time.

1. workbench.editor.limit.enabled: Enable this feature (off by default).
2. workbench.editor.limit.perEditorGroup: Whether to apply the limit per editor
    group or across all groups.
3. workbench.editor.limit.value: The maximum number of open editors (10 by 
default).