Block Template Utils: Add navigation overlay template part area const…

[scruffian]

Summary

Backports the changes from Gutenberg PR WordPress/gutenberg#74444 to rename the navigation overlay template part area.

This PR introduces the WP_TEMPLATE_PART_AREA_NAVIGATION_OVERLAY constant to define a dedicated template part area for navigation overlays, providing more specificity than the generic "overlay" naming.

Changes

• Adds new constant WP_TEMPLATE_PART_AREA_NAVIGATION_OVERLAY with value 'navigation-overlay' in src/wp-includes/block-template-utils.php

Context

The Gutenberg PR renamed the generic "overlay" template part area to "navigation-overlay" for better clarity and specificity. This backport brings that constant definition to WordPress core.

The backport is minimal since WordPress core only needs the PHP constant definition. The JavaScript changes in the Gutenberg PR are bundled with the block library and will be included when the Gutenberg packages are synced to core.

There is an accompanying PR that was an earlier version of this here: #10581

Props: getdave

Trac ticket: https://core.trac.wordpress.org/ticket/64334#ticket

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Props scruffian.

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