add JS interop stub for the JS Map class

+import 'dart:js_interop';
+
+/// A static interop stub for the `Map` class.
+///
+/// This stub is here because the `js_types` from the Dart SDK do not yet provide a `Map` equivalent:
+/// https://github.com/dart-lang/sdk/issues/54365
+///
+/// See also: https://github.com/dart-lang/sdk/issues/54365#issuecomment-1856995463
+///
+/// Object literals can be made using [jsify].
+@JS('Map')
+@staticInterop
+class JsMap<K extends JSAny, V extends JSAny> implements JSAny {
+  external factory JsMap();
+}
+
+extension JsMapExtension<K extends JSAny, V extends JSAny> on JsMap<K, V> {
+  external V? get(K key);
+  external JSVoid set(K key, V? value);
+}