Fix wasm builds with file_watcher enabled (#10589)

# Objective

- Currently, in 0.12 there is an issue that it is not possible to build
bevy for Wasm with feature "file_watcher" enabled. It still would not
compile, but now with proper explanation.
- Fixes #10507


## Solution

- Remove `notify-debouncer-full` dependency on WASM platform entirely.
- Compile with "file_watcher" feature now on platform `wasm32` gives
meaningful compile error.

---

## Changelog

### Fixed

- Compile with "file_watcher" feature now on platform `wasm32` gives
meaningful compile error.

crates/bevy_asset/Cargo.toml

@@ -38,7 +38,6 @@ parking_lot = { version = "0.12", features = ["arc_lock", "send_guard"] }
 ron = "0.8"
 serde = { version = "1", features = ["derive"] }
 thiserror = "1.0"
-notify-debouncer-full = { version = "0.3.1", optional = true }
 
 [target.'cfg(target_os = "android")'.dependencies]
 bevy_winit = { path = "../bevy_winit", version = "0.12.0" }
@@ -49,5 +48,8 @@ web-sys = { version = "0.3", features = ["Request", "Window", "Response"] }
 wasm-bindgen-futures = "0.4"
 js-sys = "0.3"
 
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+notify-debouncer-full = { version = "0.3.1", optional = true }
+
 [dev-dependencies]
 bevy_core = { path = "../bevy_core", version = "0.12.0" }

crates/bevy_asset/src/io/mod.rs

@@ -1,3 +1,10 @@
+#[cfg(all(feature = "file_watcher", target_arch = "wasm32"))]
+compile_error!(
+    "The \"file_watcher\" feature for hot reloading does not work \
+    on WASM.\nDisable \"file_watcher\" \
+    when compiling to WASM"
+);
+
 #[cfg(target_os = "android")]
 pub mod android;
 pub mod embedded;