Added helper methods to Bundles. (#17464)

Added `len`, `is_empty`, and `iter` methods to `Bundles`.

Separated out from #17331.

---------

Co-authored-by: shuo <shuoli84@gmail.com>

crates/bevy_ecs/src/bundle.rs

@@ -1473,6 +1473,21 @@ pub struct Bundles {
 }
 
 impl Bundles {
+    /// The total number of [`Bundle`] registered in [`Storages`].
+    pub fn len(&self) -> usize {
+        self.bundle_infos.len()
+    }
+
+    /// Returns true if no [`Bundle`] registered in [`Storages`].
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Iterate over [`BundleInfo`].
+    pub fn iter(&self) -> impl Iterator<Item = &BundleInfo> {
+        self.bundle_infos.iter()
+    }
+
     /// Gets the metadata associated with a specific type of bundle.
     /// Returns `None` if the bundle is not registered with the world.
     #[inline]