Decreasing the Core code Size by making the mixin an entity instead of having its own element (discussion maybe needed)

I think we can make the overall code of A-Frame shorter, hence better for loading for first time, not sure if this can affect negatively, if we make the core code of an "" accepts pointing to (referencing) any other entity to be used as its mixin.

if the issues concerning are related to the nested mixins (an entity referencing another entity through "mixin" comp that by itself is also referencing to another entity), we can add a condition to check if the referenced mixin is already an entity that referencing another entity and warn the user (who use inspector/editor) and developers (who directly code)..!

No files or tests are named. Start by tracing the core and mixin component, then examine how nested references would affect the inspector/editor. Done requires an agreed design plus validation that the change preserves behavior and reduces core code size.