Add a public constructor for `Mut<T>`

[joseph-gio]

Objective

Fix #6497.

Since it is not possible to construct an instance of Mut<T> outside of bevy_ecs, other crates that need similar behavior (such as bevy-trait-query) need to create their own duplicate implementations of this type.

Solution

Add a public constructor for Mut<T>.

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Changelog

• Added the associated function new to Mut<T>.