Details block: Add placeholder attribute

[shimotmk]

What?

It would be useful to be able to set a placeholder in the summary of Details.
Add a placeholder as shown below.

Similar
#30958
#1891

Testing Instructions

1. Open a post or page. -->
2. Insert a Details block. -->
3. Switch to the code editor and add a placeholer.

example

<!-- wp:details {"placeholder":"summary is here!"} -->
<details class="wp-block-details"><summary></summary><!-- wp:paragraph {"placeholder":"Type / to add a hidden block"} -->
<p></p>
<!-- /wp:paragraph --></details>
<!-- /wp:details -->

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: shimotmk <shimotomoki@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

A great addition. Thanks, @shimotmk!

[shimotmk]

@Mamaduka
Thank you again!