Added helpful adders for systemsets (#2366)

# Objective

- This adds a way to add `SystemSet`s to Apps.

crates/bevy_app/src/app_builder.rs

@@ -232,6 +232,10 @@ impl AppBuilder {
         self.add_startup_system_to_stage(StartupStage::Startup, system)
     }
 
+    pub fn add_startup_system_set(&mut self, system_set: SystemSet) -> &mut Self {
+        self.add_startup_system_set_to_stage(StartupStage::Startup, system_set)
+    }
+
     pub fn add_startup_system_to_stage(
         &mut self,
         stage_label: impl StageLabel,
@@ -245,6 +249,19 @@ impl AppBuilder {
         self
     }
 
+    pub fn add_startup_system_set_to_stage(
+        &mut self,
+        stage_label: impl StageLabel,
+        system_set: SystemSet,
+    ) -> &mut Self {
+        self.app
+            .schedule
+            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+                schedule.add_system_set_to_stage(stage_label, system_set)
+            });
+        self
+    }
+
     /// Adds a new [State] with the given `initial` value.
     /// This inserts a new `State<T>` resource and adds a new "driver" to [CoreStage::Update].
     /// Each stage that uses `State<T>` for system run criteria needs a driver. If you need to use