Separator: Add ability to change HTML tag

[luminuu]

What problem does this address?

Traditionally, the separator block has been tied to the <hr> element, which is semantically correct. With the work on Twenty Twenty-Four, we've discovered that there's a need for adding a separator that is not announced by screen readers, when it's just a visual separator. The current solution is not the best one in TT4 either, as it uses the headings block and the visual separator can only be seen if a heading has content, to avoid the A11Y issue of having empty headings.

References:
WordPress/twentytwentyfour#522
WordPress/twentytwentyfour#449
WordPress/twentytwentyfour#471

What is your proposed solution?

Add a HTML tag selector to specifiy the element which should be used for the seperator, like it exists at the group block (div, main, section, etc)