Add a method for converting MutUntyped -> Mut<T> (#7113)

# Objective

`MutUntyped` is a struct that stores a `PtrMut` alongside change tick metadata. Working with this type is cumbersome, and has few benefits over storing the pointer and change ticks separately.

Related: #6430 (title is out of date)

## Solution

Add a convenience method for transforming an untyped change detection pointer into its typed counterpart.

---

## Changelog

- Added the method `MutUntyped::with_type`.

crates/bevy_ecs/src/change_detection.rs

@@ -635,6 +635,17 @@ impl<'a> MutUntyped<'a> {
     pub fn as_ref(&self) -> Ptr<'_> {
         self.value.as_ref()
     }
+
+    /// Transforms this [`MutUntyped`] into a [`Mut<T>`] with the same lifetime.
+    ///
+    /// # Safety
+    /// - `T` must be the erased pointee type for this [`MutUntyped`].
+    pub unsafe fn with_type<T>(self) -> Mut<'a, T> {
+        Mut {
+            value: self.value.deref_mut(),
+            ticks: self.ticks,
+        }
+    }
 }
 
 impl<'a> DetectChanges for MutUntyped<'a> {