[Add] Basic project architecture

Assets/Minesweeper/Runtime/Core/GameStateService.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace Minesweeper.Core
+{
+    public sealed class GameStateService : IGameStateService
+    {
+        public event Action<GameState> StateChanged;
+
+        public GameState Current { get; private set; } = GameState.FieldSelection;
+
+        public void SetState(GameState state)
+        {
+            if (Current == state)
+            {
+                return;
+            }
+
+            Current = state;
+            StateChanged?.Invoke(Current);
+        }
+    }
+}