Panel: Add 40px size prop to Button

[mirka]

In preparation for #65751

What?

Adds __next40pxDefaultSize prop to the Button in the Panel component.

Testing Instructions

See the Panel story in Storybook. There should be no visual changes.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: mirka <[email protected]>
Co-authored-by: tyxla <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Flaky tests detected in 89847b6.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11592952135
📝 Reported issues:

• [Flaky Test] should be able to open the tags panel and create a new tag #59638 in /test/e2e/specs/editor/various/taxonomies.spec.js

[tyxla]

Panel title button is unchanged 👍 🚀