Add screen_admin_context filter

[geekofshire]

This PR introduces a new filter, screen_admin_context, to the WP_Screen class, enabling developers to modify the $in_admin property dynamically. This property is currently locked to specific values (such as 'site' for regular site admin and 'network' for network admin), making it challenging to accurately identify additional custom administration panels introduced by plugins.

Trac ticket: https://core.trac.wordpress.org/ticket/37445

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