Post date block: to display post date and modified date side by side.

[iidastudio]

I want to display the post date and modified date side by side.
To do this, I need the option of not displaying the modified date on the front if it has never been modified.
I think we also need to add a class to distinguish each of them.

[carolinan]

If there is consensus to change the behavior of the modified date, this PR should solve it #46839
But we need to decide what we consider a modification, because if we create a post, and spend say half an hour writing the content, WordPress will save a modified date: I assume it is common to not publish the post immediately.