Add a Default value for font settings

Currently, when using the Font settings - family and style, you can select an empty field that corresponds to the Default value. We should call it "default" instead of having it empty and clickable:
<img width="611" alt="image" src="https://github.com/Codeinwp/visualizer/assets/86592992/34ea21e2-5e7c-487a-aade-9c08bad69047">
