Change relevant methods in World and co to return ResMut

[lynn-lumen]

Objective

• Fixes #11765

Solution

• Changed the current implementations to use ResMut instead of Mut

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Changelog

• In World: resource_mut, get_resource_mut, get_resource_or_insert_with now return ResMut / Option<ResMut> and resource_scopenow accepts a function parameter of type impl FnOnce(&mut World, ResMut<R>) -> U
• In UnsafeWorldCell: get_resource_mut now returns Option<ResMut>
• Changed internal uses of these methods to use ResMut aswell.
• ResMut<R> now implements From<Mut<R: Resource>>

Migration Guide

• For most use cases this should not require any action besides changing Mut<T> to ResMut<T> since they are very similar to each other and Mut<T> implements From<ResMut<'w, T>> for Mut<'w, T>. But you could just call .into() if any problems arise.