add methods to *EntityMut to UnsafeEntityCell as an escape hatch (#21234)

# Objective

- Sometimes you want to break mutability rules to get multiple mutable
components off of an entity.

## Solution

- Allow converting the EntityMut, FilteredEntityMut, and EntityMutExcept
into an UnsafeEntityCell.
https://docs.rs/bevy/latest/bevy/ecs/world/unsafe_world_cell/struct.UnsafeEntityCell.html

## Testing

- haven't really tested, but changes are straightforward

Author: hymm

crates/bevy_ecs/src/world/entity_ref.rs

@@ -479,6 +479,11 @@ impl<'w> EntityMut<'w> {
         EntityRef::from(self)
     }
 
+    /// Get access to the underlying [`UnsafeEntityCell`]
+    pub fn as_unsafe_entity_cell(&mut self) -> UnsafeEntityCell<'_> {
+        self.cell
+    }
+
     /// Returns the [ID](Entity) of the current entity.
     #[inline]
     #[must_use = "Omit the .id() call if you do not need to store the `Entity` identifier."]
@@ -3873,6 +3878,11 @@ impl<'w, 's> FilteredEntityMut<'w, 's> {
         FilteredEntityRef::from(self)
     }
 
+    /// Get access to the underlying [`UnsafeEntityCell`]
+    pub fn as_unsafe_entity_cell(&mut self) -> UnsafeEntityCell<'_> {
+        self.entity
+    }
+
     /// Returns the [ID](Entity) of the current entity.
     #[inline]
     #[must_use = "Omit the .id() call if you do not need to store the `Entity` identifier."]
@@ -4446,6 +4456,11 @@ where
         EntityRefExcept::from(self)
     }
 
+    /// Get access to the underlying [`UnsafeEntityCell`]
+    pub fn as_unsafe_entity_cell(&mut self) -> UnsafeEntityCell<'_> {
+        self.entity
+    }
+
     /// Gets access to the component of type `C` for the current entity. Returns
     /// `None` if the component doesn't have a component of that type or if the
     /// type is one of the excluded components.