Include the option to select named instances defined in variable fonts

[chrissimpkins]

This is an excellent suggestion from Stephen Nixon. It might be helpful to pull the fvar and/or STAT table defined named instance data in variable fonts and provide a fast/easy way to expose those instances to users for instancing. I really like this idea.

These are likely to be the most commonly needed instances in production fonts. This tool has potential use cases during development before these named instances are defined (e.g. designer creates temp design axes and user slices the designspace to review in static font format as defaults are being defined) so we will need to fall back gracefully if there are no named instances available.