fix: preserve entity references during template revert all

[willeastcott]

Summary

• Fixes pressing Revert All on a template instance incorrectly clearing entity attribute references (e.g. script attributes of type entity) that point to entities within the template hierarchy
• Adds a preserveEntityReferences option to Entity.delete() / Entities.delete() / deleteEntities() that skips reference nullification during deletion
• Uses this option in the templates:revertAll redo and undo paths, where entities are immediately re-created with the same resource_id values

Root Cause

When Revert All deletes the template instance, deleteEntities() finds all entity references in the scene and sets any pointing to the deleted entities to null. The entities are then re-created with the same IDs via instantiateTemplate(), but by that point the references have already been permanently lost.

Fix

Skip reference nullification when the caller knows the entities will be immediately re-created with identical resource_id values, which is always the case during template revert.

Fixes #691

Test plan

• Create a template and place it in the hierarchy
• Create a script with an entity attribute and assign it to another entity
• Assign the templated entity to that entity attribute
• Modify something inside the template instance
• Press Revert All
• Verify the entity attribute still references the templated entity
• Undo the revert and verify the reference is still intact