chore: scaffold the Serverpod 4 build

Deviations from a stock `serverpod create`:
- Dropped Flutter web support, for two reasons:
  - Appflowy editor had issues in the past, potentially related to the
    Flutter-Web IME bridge. Upstream has also open bugs in relation to
that:
    - AppFlowy-IO/appflowy-editor#1029 [Bug][Web] Unwanted space on click
       and overall space bugs (open)
    - AppFlowy-IO/appflowy-editor#782 [Bug] Editor auto-scrolling when
       pressing space too quickly if placed inside a scrollable (open)
  - Offline-first is a required feature. serverpod_offline_sync's SQLite
    support in browsers, maybe via sqlite3.wasm + OPFS, is not
documented.
- SDK constraints raised to ^3.13.0, instead of ^3.12.2, so enjoy the
  newest language features.
- Pinned to Flutter 3.47.4 via FVM.

Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/widget_test.dart

@@ -0,0 +1,15 @@
+// This is a basic Flutter widget test.
+//
+// To perform an interaction with a widget in your test, use the WidgetTester
+// utility that Flutter provides. For example, you can send tap and scroll
+// gestures. You can also use WidgetTester to find child widgets in the widget
+// tree, read text, and verify that the values of widget properties are correct.
+
+// import 'package:flutter/material.dart';
+// import 'package:flutter_test/flutter_test.dart';
+//
+// import 'package:PROJECTNAME_flutter/main.dart';
+
+void main() {
+  // Add your app tests here
+}