Add table caption as a type of thing (so we can style it properly and keep it with the table)

Duplicated in Wax repo: wax/wax-prosemirror#51 (closed)

Context

Currently you can add a table, you could add a caption below it manually as a paragraph, but often it will separate across pages from the table which is not a good look at all. Tables need captions for reference purposes, especially in STEM subjects.

Proposal

When you add an image, when you click on the image you get a caption space below it but don't have to add a caption.

Design

Similar to image caption: table caption block appears below table automatically and can be deleted.

Alternatively add 'Add table caption' to table options allowing one caption to be added per table.

Implementation (if applicable)

Alternative approaches (if applicable)