[HTML5] Replace XMLHttpRequest with Fetch.

This has some advantages:
- Streaming/chunked response support.
- Broader headers support.

platform/javascript/SCsub

@@ -17,7 +17,7 @@ sys_env.AddJSLibraries(
     [
         "js/libs/library_godot_audio.js",
         "js/libs/library_godot_display.js",
-        "js/libs/library_godot_http_request.js",
+        "js/libs/library_godot_fetch.js",
         "js/libs/library_godot_os.js",
         "js/libs/library_godot_runtime.js",
     ]