Improve the preview states for blocks that require entities

[gigitux]

Some blocks can be used by different postTypes, taxonomies/terms, etc, and the placeholder could use that information to show better descriptions/texts.

For example, the Post Excerpt preview could be improved if it was possible to get the post type in the block #48964

Also, for other blocks, it could be enough to infer the templateSlug. For example, could this approach be a good idea for the Post Title? #48963

Tasks

• Post Title: Displays the post type name as preview in the editor #48963
• Post Excerpt: Displays the post type name as preview in the editor #48964
• Comments Title: Displays the post type name as preview in the editor #50169

[draganescu]

I added the needs desgin feedback and needs copy review for the linked issues.