Rename observe to observe_entity on EntityWorldMut

[kristoff3r]

Objective

The current observers have some unfortunate footguns where you can end up confused about what is actually being observed. For apps you can chain observe like app.observe(..).observe(..) which works like you would expect, but if you try the same with world the first observe() will return the EntityWorldMut for the created observer, and the second observe() will only observe on the observer entity. It took several hours for multiple people on discord to figure this out, which is not a great experience.

Solution

Rename observe on entities to observe_entity. It's slightly more verbose when you know you have an entity, but it feels right to me that observers for specific things have more specific naming, and it prevents this issue completely.

Another possible solution would be to unify observe on App and World to have the same kind of return type, but I'm not sure exactly what that would look like.

Testing

Simple name change, so only concern is docs really.

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Migration Guide

The observe() method on entities has been renamed to observe_entity() to prevent confusion about what is being observed in some cases.