Add Spectre.Cli to Spectre.Console

* Renames Spectre.Cli to Spectre.Console.Cli.
* Now uses Verify with Spectre.Console.Cli tests.
* Removes some duplicate definitions.

Closes #168

src/Spectre.Console/Cli/FlagValue.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Globalization;
+
+namespace Spectre.Console.Cli
+{
+    /// <summary>
+    /// Implementation of a flag with an optional value.
+    /// </summary>
+    /// <typeparam name="T">The flag's element type.</typeparam>
+    public sealed class FlagValue<T> : IFlagValue
+    {
+        /// <summary>
+        /// Gets or sets a value indicating whether or not the flag was set or not.
+        /// </summary>
+        public bool IsSet { get; set; }
+
+#pragma warning disable CS8618 // Non-nullable field is uninitialized. Consider declaring as nullable.
+        /// <summary>
+        /// Gets or sets the flag's value.
+        /// </summary>
+        public T Value { get; set; }
+#pragma warning restore CS8618 // Non-nullable field is uninitialized. Consider declaring as nullable.
+
+        /// <inheritdoc/>
+        Type IFlagValue.Type => typeof(T);
+
+        /// <inheritdoc/>
+        object? IFlagValue.Value
+        {
+            get => Value;
+            set
+            {
+#pragma warning disable CS8601 // Possible null reference assignment.
+                Value = (T)value;
+#pragma warning restore CS8601 // Possible null reference assignment.
+            }
+        }
+
+        /// <inheritdoc/>
+        public override string ToString()
+        {
+            var flag = (IFlagValue)this;
+            if (flag.Value != null)
+            {
+                return string.Format(
+                    CultureInfo.InvariantCulture,
+                    "Set={0}, Value={1}",
+                    IsSet,
+                    flag.Value.ToString());
+            }
+
+            return string.Format(
+                CultureInfo.InvariantCulture,
+                "Set={0}", IsSet);
+        }
+    }
+}