Added Vec append to BufferVec - Issue #3531 (#8575)

# Objective

- Fixes #3531 

## Solution

- Added an append wrapper to BufferVec based on the function signature
for vec.append()

---

First PR to Bevy. I didn't see any tests for other BufferVec methods
(could have missed them) and currently this method is not used anywhere
in the project. Let me know if there are tests to add or if I should
find somewhere to use append so it is not dead code. The issue mentions
implementing `truncate` and `extend` which were already implemented and
merged
[here](https://github.com/bevyengine/bevy/pull/6833/files#diff-c8fb332382379e383f1811e30c31991b1e0feb38ca436c357971755368012ced)

crates/bevy_render/src/render_resource/buffer_vec.rs

@@ -75,6 +75,10 @@ impl<T: Pod> BufferVec<T> {
         index
     }
 
+    pub fn append(&mut self, other: &mut BufferVec<T>) {
+        self.values.append(&mut other.values);
+    }
+
     pub fn set_label(&mut self, label: Option<&str>) {
         let label = label.map(str::to_string);