Add support for "Save as" in notebook menu of JupyterLite

[FrancescAlted]

Although "Save as" does not work, I just discovered that the "Rename" capability has a similar behaviour; as it tries to changes the name in a 'local' filesystem, it seems like it is able to create a new notebook with the new name, but the old one is still kept (probably because it could not be found for deletion). I wonder if we can take advantage of this 'capability' to implement the "Save as" functionality.