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/BuyersTests/BuyersTests.cs

@@ -0,0 +1,19 @@
+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotal()
+        {
+            // Anke: 2 flavours x 2 scoops x base prices (3 + 2) = 10.
+            // Red or flaky — why? (PriceList adds random surge pricing;
+            // it stands for the external pricing service.)
+            var buyers = new Buyers();
+            Assert.AreEqual(10, buyers.ShoppingListTotal("Anke"));
+        }
+    }
+}