Add `is_post_publicly_viewable` filter

[peterwilsoncc]

Related to #49628, #54375.

Add a filter to the is_post_publicly_viewable() function to allow theme and plugin developers to override the default value.

In some circumstances a developer may require the checks use different conditions to the default.

[rafiahmedd]

I think developers can use is_post_status_viewable & is_post_type_viewable filter hook for modifications.