Add support for selecting prompt items

Closes #447

test/Spectre.Console.Tests/Unit/Prompt/MultiSelectionPromptTests.cs

@@ -0,0 +1,86 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using Shouldly;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public sealed class MultiSelectionPromptTests
+    {
+        private class CustomItem
+        {
+            public int X { get; set; }
+            public int Y { get; set; }
+
+            public class Comparer : IEqualityComparer<CustomItem>
+            {
+                public bool Equals(CustomItem x, CustomItem y)
+                {
+                    return x.X == y.X && x.Y == y.Y;
+                }
+
+                public int GetHashCode([DisallowNull] CustomItem obj)
+                {
+                    throw new NotImplementedException();
+                }
+            }
+        }
+
+        [Fact]
+        public void Should_Not_Mark_Item_As_Selected_By_Default()
+        {
+            // Given
+            var prompt = new MultiSelectionPrompt<int>();
+
+            // When
+            var choice = prompt.AddChoice(32);
+
+            // Then
+            choice.IsSelected.ShouldBeFalse();
+        }
+
+        [Fact]
+        public void Should_Mark_Item_As_Selected()
+        {
+            // Given
+            var prompt = new MultiSelectionPrompt<int>();
+            var choice = prompt.AddChoice(32);
+
+            // When
+            prompt.Select(32);
+
+            // Then
+            choice.IsSelected.ShouldBeTrue();
+        }
+
+        [Fact]
+        public void Should_Mark_Custom_Item_As_Selected_If_The_Same_Reference_Is_Used()
+        {
+            // Given
+            var prompt = new MultiSelectionPrompt<CustomItem>();
+            var item = new CustomItem { X = 18, Y = 32 };
+            var choice = prompt.AddChoice(item);
+
+            // When
+            prompt.Select(item);
+
+            // Then
+            choice.IsSelected.ShouldBeTrue();
+        }
+
+        [Fact]
+        public void Should_Mark_Custom_Item_As_Selected_If_A_Comparer_Is_Provided()
+        {
+            // Given
+            var prompt = new MultiSelectionPrompt<CustomItem>(new CustomItem.Comparer());
+            var choice = prompt.AddChoice(new CustomItem { X = 18, Y = 32 });
+
+            // When
+            prompt.Select(new CustomItem { X = 18, Y = 32 });
+
+            // Then
+            choice.IsSelected.ShouldBeTrue();
+        }
+    }
+}