Add flutter as submodule

.gitmodules

@@ -2,3 +2,7 @@
 	path = thirdparty/sentry-dart
 	url = https://github.com/ente-io/sentry-dart.git
 	branch = sentry_flutter_ente
+[submodule "flutter"]
+	path = flutter
+	url = https://github.com/flutter/flutter.git
+	branch = stable

flutter

@@ -0,0 +1 @@
+Subproject commit b06b8b2710955028a6b562f5aa6fe62941d6febf