[feature] a way to "pull" entities on the map into template instances

[mfaizsyahmi]

Let's say I pass a keyvalue to a macro_insert whose value is the name of an entity. Then inside the template a custom mess point entity, or a _mess custom keyvalue, takes that value resulting in an entity already present in the map, with that name, being "pulled" into the instance and replaces the point entity.

A use case of this are templates having ambient_generics or env_sprites. Rather than exposing every keyvalue and flag for those entities to a .ted entity or a macro_insert's keyvalues for maximum customizability, you just give those a name of an external entity to be used in its stead. A sort of BYOB but for entities, if you will.