Fix file_watcher feature hanging indefinitely (#10585)

# Objective

Fix the `bevy_asset/file_watcher` feature in practice depending on
multithreading, while not informing the user of it.

**As I understand it** (I didn't check it), the file watcher feature
depends on spawning a concurrent thread to receive file update events
from the `notify-debouncer-full` crate. But if multithreading is
disabled, that thread will never have time to read the events and
consume them.

- Fixes #10573 


## Solution

Add a `compile_error!` causing compilation failure if `file_watcher` is
enabled while `multi-threaded` is disabled.

This is considered better than adding a dependency on `multi-threaded`
on the `file_watcher`, as (according to @mockersf) toggling on/off
`multi-threaded` has a non-zero chance of changing behavior. And we
shouldn't implicitly change behavior. A compilation failure prevents
compilation of code that is invalid, while informing the user of the
steps needed to fix it.

Author: nicopap

crates/bevy_asset/src/lib.rs

@@ -52,6 +52,13 @@ use bevy_log::error;
 use bevy_reflect::{FromReflect, GetTypeRegistration, Reflect, TypePath};
 use std::{any::TypeId, sync::Arc};
 
+#[cfg(all(feature = "file_watcher", not(feature = "multi-threaded")))]
+compile_error!(
+    "The \"file_watcher\" feature for hot reloading requires the \
+    \"multi-threaded\" feature to be functional.\n\
+    Consider either disabling the \"file_watcher\" feature or enabling \"multi-threaded\""
+);
+
 /// Provides "asset" loading and processing functionality. An [`Asset`] is a "runtime value" that is loaded from an [`AssetSource`],
 /// which can be something like a filesystem, a network, etc.
 ///