Post Excerpt: Specify paragraph as post excerpt tag name

[mikachan]

Description

This is a small PR to change the tag for the post excerpt in the editor from a div to a p. This is to bring the markup more in line with what is output on the front end, which is a p inside a div, like this:

<div class="wp-block-post-excerpt">
     <p class="wp-block-post-excerpt__excerpt">...

This also fixes potential styling inconsistencies between the editor and the front end with styles that are applied to paragraph tags.

How has this been tested?

I've manually tested this using emptytheme, inserting a post excerpt and then ensuring the element changes to a paragraph in the editor.

Screenshots

Before	After

Types of changes

Enhancement

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).