Allow default settings for .ipynb vs. .dib and default setting for default new notebook language

[claudiaregio]

When creating a new notebook through the command palette, a user always has to select whether they want to use the .ipynb format or the .dib format.

Would be nice if there were a setting that allowed users to select a default file type such that they don't have to select every time they create a new notebook. For users who stick to one file type (my assumption is most do this), it would reduce the creation of notebooks by one click.

Currently:

With setting:

[jonsequitur]

The same would be useful for the default language.