Add ability to visually replace separator block with an image

[cbirdsong]

What problem does this address?

When laying out a heavily art-directed post/page it's often nice to have a custom-designed separator element to make the whole thing feel a bit more cohesive. The current ways to accomplish this in Gutenberg are:

• Changing the color of the separator block. This is a nice option, but it's obviously pretty limited.
• Using an image block. This works visually but is not accessible, since it carries no semantic meaning for screen readers or reader view. As far as they're concerned it's just another image.

What is your proposed solution?

Add the ability to replace the separator block with an image, implemented either using a background image on the <hr> tag or an <img> tag with role="separator". Sara Soueidan has a great blog post demonstrating these two approaches and going over the pros/cons of each one: https://www.sarasoueidan.com/blog/horizontal-rules/