Add pop method for List trait. (bevyengine#5797)

# Objective

- The reflection `List` trait does not have a `pop` function.
- Popping elements off a list is a common use case and is almost always supported by `List`-like types.

## Solution

- Add the `pop()` method to the `List` trait and add the appropriate implementations of this function.

## Migration Guide

- Any custom type that implements the `List` trait will now need to implement the `pop` method.


Co-authored-by: Carter Anderson <[email protected]>

crates/bevy_reflect/src/impls/smallvec.rs

@@ -54,6 +54,10 @@ where
         });
         SmallVec::push(self, value);
     }
+
+    fn pop(&mut self) -> Option<Box<dyn Reflect>> {
+        self.pop().map(|value| Box::new(value) as Box<dyn Reflect>)
+    }
 }
 
 impl<T: smallvec::Array + Send + Sync + 'static> Reflect for SmallVec<T>

crates/bevy_reflect/src/impls/std.rs

@@ -137,6 +137,10 @@ impl<T: FromReflect> List for Vec<T> {
         });
         Vec::push(self, value);
     }
+
+    fn pop(&mut self) -> Option<Box<dyn Reflect>> {
+        self.pop().map(|value| Box::new(value) as Box<dyn Reflect>)
+    }
 }
 
 impl<T: FromReflect> Reflect for Vec<T> {

crates/bevy_reflect/src/list.rs

@@ -15,6 +15,9 @@ pub trait List: Reflect + Array {
     /// Appends an element to the list.
     fn push(&mut self, value: Box<dyn Reflect>);
 
+    /// Removes the last element from the list (highest index in the array) and returns it, or [`None`] if it is empty.
+    fn pop(&mut self) -> Option<Box<dyn Reflect>>;
+
     /// Clones the list, producing a [`DynamicList`].
     fn clone_dynamic(&self) -> DynamicList {
         DynamicList {
@@ -151,6 +154,10 @@ impl List for DynamicList {
         DynamicList::push_box(self, value);
     }
 
+    fn pop(&mut self) -> Option<Box<dyn Reflect>> {
+        self.values.pop()
+    }
+
     fn clone_dynamic(&self) -> DynamicList {
         DynamicList {
             name: self.name.clone(),