Added Symfony AssetMapper.

importmap.php

@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * Returns the importmap for this application.
+ *
+ * - "path" is a path inside the asset mapper system. Use the
+ *     "debug:asset-map" command to see the full list of paths.
+ *
+ * - "entrypoint" (JavaScript only) set to true for any module that will
+ *     be used as an "entrypoint" (and passed to the importmap() Twig function).
+ *
+ * The "importmap:require" command can be used to add new entries to this file.
+ */
+return [
+    'app' => [
+        'path' => './assets/app.js',
+        'entrypoint' => true,
+    ],
+];