Issue #2877633 by larowlan: Add a 'is parent of' argument plugin

config/schema/entity_hierarchy.views.schema.yml

entity_hierarchy.views.inc

...ess/src/Plugin/AccessControlHierarchy/EntityHierarchy.php

...ugin/views/argument/EntityHierarchyArgumentPluginBase.php

src/Plugin/views/argument/HierarchyIsChildOfEntity.php

...lugin/views/argument/HierarchyIsChildOfEntityRevision.php

src/Plugin/views/argument/HierarchyIsParentOfEntity.php

...ugin/views/argument/HierarchyIsParentOfEntityRevision.php

...nstall/views.view.entity_hierarchy_test_children_view.yml

tests/src/Kernel/ViewsIntegrationTest.php