Fixes #22080, fixes #23762 (by stopping those crashes from occurring when a post content block exists.)

Related to #24006.

Description

• Adds Placeholder content for Post Title block which displays when editing wp_template.
• Adds placeholder for post content block, which displays when the root post is different from the post content (to avoid a crash. This works because there is no root post in FSE, but there is in all the other editors.)
• Restructuring of how we pull props in the Query Loop component. This would break with a block error when loaded in the old editor due to lack of expected props being supplied through context. Now it just finds the posts without a supplied query.

Although it will show all posts retrieved from the entity records as there is no query to control or limit them. 🤔

How has this been tested?

locally in edit site, edit post (while editing posts and templates)

Screenshots

For the Singular template, instead of looking totally broken we get:

(note that the post content placeholder has since been updated to look like this:)

And for the Index template the broken query loop block:

Shows posts:

Types of changes

Bug fix, enhancement

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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.