Slice with lambda: add C# exercise projects and export

- Add SliceWithLambda.slnx + csproj scaffolding for ideas/slice-with-lambda/
  exercise/csharp (examples project + xUnit solution/test project)
- Add exercises/slice-with-lambda (C# version of the prefactored-longrange
  exercise with Buyers and SliceWithLambda projects)
- Add export/slice-with-lambda (nested git repo with exercise export)
- Ignore bin/ and obj/ build output at repo root

exercises/slice-with-lambda/tests/BuyersSolutionTests/BuyersTests.cs

@@ -0,0 +1,20 @@
+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotalUsesThePricingWePassIn()
+        {
+            // pure pricing function: everything costs 2.
+            // Anke: 2 flavours x 2 scoops x 2 = 8 — deterministic, glanceable.
+            var buyers = new Buyers();
+            var anke = new Buyer("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla });
+            Assert.AreEqual(8, buyers.ShoppingListTotal(anke, _ => 2));
+        }
+    }
+}