Add a getter for asset watching status on AssetServer (#11578)

# Objective

- Closes #11490.
- Allow retrieving the current asset watch behavior from the
`AssetServer`.

## Solution

- Add the corresponding getter. (also fixes some trailing whitespace).

A corresponding helper could also be added on the `AssetPlugin` struct
(returning `self.watch_for_changes_override.unwrap_or(cfg!(feature =
"watch"))`), but it seems it isn't a current practice to have actual
methods on the plugin structs appart from the `Plugin` impl.

---

## Changelog

### Added

Added `watching_for_changes` getter on `AssetServer`.

---------

Co-authored-by: Mateusz Wachowiak <mateusz_wachowiak@outlook.com>

crates/bevy_asset/src/server/mod.rs

@@ -128,6 +128,11 @@ impl AssetServer {
         self.data.sources.get(source.into())
     }
 
+    /// Returns true if the [`AssetServer`] watches for changes.
+    pub fn watching_for_changes(&self) -> bool {
+        self.data.infos.read().watching_for_changes
+    }
+
     /// Registers a new [`AssetLoader`]. [`AssetLoader`]s must be registered before they can be used.
     pub fn register_loader<L: AssetLoader>(&self, loader: L) {
         let mut loaders = self.data.loaders.write();