Set up C# solution

src/Main.cs

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+namespace GameJamDungeon;
+
+using Godot;
+
+#if DEBUG
+using System.Reflection;
+#endif
+
+// This entry-point file is responsible for determining if we should run tests.
+//
+// If you want to edit your game's main entry-point, please see Game.tscn and
+// Game.cs instead.
+
+public partial class Main : Node2D
+{
+
+  public override void _Ready()
+  {
+    // If we don't need to run tests, we can just switch to the game scene.
+    CallDeferred("RunScene");
+  }
+
+  private void RunScene()
+    => GetTree().ChangeSceneToFile("res://src/app/App.tscn");
+}

src/Main.tscn

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+[gd_scene load_steps=2 format=3 uid="uid://d1gjaijijd5ot"]
+
+[ext_resource type="Script" path="res://src/Main.cs" id="1_prpoe"]
+
+[node name="Node2D" type="Node2D"]
+script = ExtResource("1_prpoe")

src/app/App.tscn

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+[gd_scene format=3 uid="uid://cagfc5ridmteu"]
+
+[node name="App" type="Node"]