Add interactive prompts for selecting values

* Adds SelectionPrompt
* Adds MultiSelectionPrompt

Closes #210

src/Spectre.Console/Widgets/Prompt/MultiSelectionPrompt.cs

@@ -0,0 +1,112 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents a list prompt.
+    /// </summary>
+    /// <typeparam name="T">The prompt result type.</typeparam>
+    public sealed class MultiSelectionPrompt<T> : IPrompt<List<T>>
+    {
+        /// <summary>
+        /// Gets or sets the title.
+        /// </summary>
+        public string? Title { get; set; }
+
+        /// <summary>
+        /// Gets the choices.
+        /// </summary>
+        public List<T> Choices { get; }
+
+        /// <summary>
+        /// Gets or sets the converter to get the display string for a choice. By default
+        /// the corresponding <see cref="TypeConverter"/> is used.
+        /// </summary>
+        public Func<T, string>? Converter { get; set; } = TypeConverterHelper.ConvertToString;
+
+        /// <summary>
+        /// Gets or sets the page size.
+        /// Defaults to <c>10</c>.
+        /// </summary>
+        public int PageSize { get; set; } = 10;
+
+        /// <summary>
+        /// Gets or sets a value indicating whether or not
+        /// at least one selection is required.
+        /// </summary>
+        public bool Required { get; set; } = true;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MultiSelectionPrompt{T}"/> class.
+        /// </summary>
+        public MultiSelectionPrompt()
+        {
+            Choices = new List<T>();
+        }
+
+        /// <inheritdoc/>
+        public List<T> Show(IAnsiConsole console)
+        {
+            if (!console.Capabilities.SupportsInteraction)
+            {
+                throw new NotSupportedException(
+                    "Cannot show multi selection prompt since the current " +
+                    "terminal isn't interactive.");
+            }
+
+            if (!console.Capabilities.SupportsAnsi)
+            {
+                throw new NotSupportedException(
+                    "Cannot show multi selection prompt since the current " +
+                    "terminal does not support ANSI escape sequences.");
+            }
+
+            var converter = Converter ?? TypeConverterHelper.ConvertToString;
+
+            var list = new RenderableMultiSelectionList<T>(console, Title, PageSize, Choices, converter);
+            using (new RenderHookScope(console, list))
+            {
+                console.Cursor.Hide();
+                list.Redraw();
+
+                while (true)
+                {
+                    var key = console.Input.ReadKey(true);
+                    if (key.Key == ConsoleKey.Enter)
+                    {
+                        if (Required && list.Selections.Count == 0)
+                        {
+                            continue;
+                        }
+
+                        break;
+                    }
+
+                    if (key.Key == ConsoleKey.Spacebar)
+                    {
+                        list.Select();
+                        list.Redraw();
+                        continue;
+                    }
+
+                    if (list.Update(key.Key))
+                    {
+                        list.Redraw();
+                    }
+                }
+            }
+
+            list.Clear();
+            console.Cursor.Show();
+
+            return list.Selections
+                .Select(index => Choices[index])
+                .ToList();
+        }
+    }
+}