drop info locks in single threaded (#15522)

# Objective

addresses half of issue #15508
avoid asset server deadlock when `multi_threaded` feature is not
enabled.

## Solution

drop the locks in the single-threaded case.

the lock is still held with the `multi-threaded` feature enabled to
avoid re-locking to insert the load task. i guess this might possibly
cause issues on single-core machines ... is that something we should
worry about?

---------

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crates/bevy_asset/src/server/mod.rs

@@ -420,6 +420,10 @@ impl AssetServer {
  infos: &mut AssetInfos,
  guard: G,
  ) {
+ // drop the lock on `AssetInfos` before spawning a task that may block on it in single-threaded
+ #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
+ drop(infos);
+
  let owned_handle = handle.clone();
  let server = self.clone();
  let task = IoTaskPool::get().spawn(async move {
@@ -469,6 +473,11 @@ impl AssetServer {
  HandleLoadingMode::Request,
  meta_transform,
  );
+
+ // drop the lock on `AssetInfos` before spawning a task that may block on it in single-threaded
+ #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
+ drop(infos);
+
  if !should_load {
  return handle;
  }
@@ -778,6 +787,11 @@ impl AssetServer {
  let mut infos = self.data.infos.write();
  let handle =
  infos.create_loading_handle_untyped(TypeId::of::<A>(), core::any::type_name::<A>());
+
+ // drop the lock on `AssetInfos` before spawning a task that may block on it in single-threaded
+ #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
+ drop(infos);
+
  let id = handle.id();
 
  let event_sender = self.data.asset_event_sender.clone();