API: Support CSS for Plugin Output

It would be nice to be able to define CSS that will be applied to any plugin output. This seems a super set of the Table Cell Visualization request as this would be able to cover that.

This can be problematic as it can mess with the overall UI and conforming to any color theme applied might be difficult. For colors anyway there could be CSS variables for any of the colors that the theme has applied.

As I'm typing up the use cases I can see right now most are relatively basic formatting options, so to prevent opening up the whole CSS world if it's preferable to put in requests on the Amplenote side for each of them I can do that.

Example use cases
- For the Calendar plugin I wish that I could specify a smaller font size as the width can get a bit unwieldy if links to jots are applied (with the icon that is applied to those links).
- Support positioning of elements (e.g. small icons placed in the Calendar day cell)
- Formatting table header rows
- Vertical alignment for table cells