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/Internal/Collections/IMultiMap.cs

@@ -0,0 +1,14 @@
+namespace Spectre.Console.Cli.Internal
+{
+    /// <summary>
+    /// Representation of a multi map.
+    /// </summary>
+    internal interface IMultiMap
+    {
+        /// <summary>
+        /// Adds a key and a value to the multi map.
+        /// </summary>
+        /// <param name="pair">The pair to add.</param>
+        void Add((object? Key, object? Value) pair);
+    }
+}