Issue #2919756 by Musa.thomas: All entity support

Add support for all entities instead of just “node” content types, and introduce hook_addtoany_entity_types_alter() to alter which entities the addtoany extra pseudo-field is available for.

Make the addtoany extra pseudo-field available for “comment” entity by default.

Create addtoany.api.php to document the new hook and future hooks.

Replace “node” references with “entity” references in function names.