Admin: Automatically enqueue postbox scripts for custom meta boxes

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/17704

This PR resolves trac ticket #17704 by ensuring that the postbox script is automatically enqueued and initialized on custom admin pages that utilize add_meta_box().

Previously, custom admin screens rendering meta boxes would lack drag-and-drop, collapse/expand toggle, and Screen Options functionality unless the plugin author manually enqueued the postbox script and initialized postboxes.add_postbox_toggles().

Testing

Run the tests using -

npm run test:php -- --filter=Tests_Admin_IncludesScreen

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