MenuItem: Add 40px size prop on Button

[mirka]

In preparation for #65751

What?

Adds __next40pxDefaultSize prop to the Button in MenuItem.

Testing Instructions

See the MenuItem stories in Storybook. The heights should now be 40px instead of 36px.

(These changes mostly won't be seen in the app until we update the styles for DropdownMenu, which is currently overriding the MenuItem styles 😅)

[github-actions]

Flaky tests detected in 34a50fa.
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/11593300372
📝 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

[github-actions]

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

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