Allow 'Main Page Content' block to use a specific display mode

Description of the need If you add an Existing Content block to a layout, you have the option of selecting a specific display mode. This option is NOT available on the "Main Page Content" block and I think it should be.

A user just pointed this out in the forum: https://forum.backdropcms.org/forum/adding-and-using-custom-display-mode...

Proposed solution

I assume that part of the problem is that the main page content is not always a node of a specific content types. Display modes are specific to content types. However, just as you can only select specific field blocks if you have a visibility condition that specifies a specific node type, we could only allow this option under the same circumstances.

@klonos makes this case here: https://github.com/backdrop/backdrop-issues/issues/3530#issuecomment-581...

Alternatives that have been considered

https://github.com/backdrop/backdrop-issues/issues/3530

Additional information Add any other information or screenshots that would help.