add Res::clone (#4109)

# Objective
Make `Res` cloneable
## Solution
Add an associated fn `clone(self: &Self) -. Self` instead of `Copy + Clone` trait impls to avoid `res.clone()` failing to clone out the underlying `T`

crates/bevy_ecs/src/system/system_param.rs

@@ -291,6 +291,17 @@ where
 }
 
 impl<'w, T: Resource> Res<'w, T> {
+    // no it shouldn't clippy
+    #[allow(clippy::should_implement_trait)]
+    pub fn clone(this: &Self) -> Self {
+        Self {
+            value: this.value,
+            ticks: this.ticks,
+            last_change_tick: this.last_change_tick,
+            change_tick: this.change_tick,
+        }
+    }
+
     /// Returns `true` if the resource was added after the system last ran.
     pub fn is_added(&self) -> bool {
         self.ticks.is_added(self.last_change_tick, self.change_tick)