table-group-divider for top and bottom

[Xorfor]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Currently the class table-group-divider adds a thicker border, darker between table groups, by changing border-top. I also like to add a thicker border at the bottom of a table. This makes it more clear where a table stops and the next table starts. Of course I can do this by creating a class. But I suggest to rename the current class table-group-divider to table-group-divider-top, and also add a class table-group-divider-bottom which will add a thicker border by changing border-bottom.

Motivation and context

This makes it more clear where a table stops and the next table starts.