Introduced MapImplicitAreaControllerRoute() overload for mapping all areas

The existing `MapImplictAreaControllerRoute()` extension method recently introduced (8a65c95) requires, at minimum, an `areaName` parameter, and thus is limited to setting up routes for one individual area at a time.

This extension doesn't (currently) accept any parameters, and will instead map _all_ areas. It does this by implementing the new `TopicRouteValueTransformer` (865f26a) to automatically set the `controller` routing variable to the `area` name, which is the convention we most frequently follow.

Be aware that, as with the recently introduced `MapTopicAreaRoute()` (762d229), this implementation trips of a feature limitation of ASP.NET Core 3.0 which prevents e.g. `@Url.Action()` references from correctly returning values (see dotnet/aspnetcore#16965). Hopefully that will be resolved in ASP.NET 4.0.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/ServiceCollectionExtensions.cs

@@ -187,6 +187,28 @@ public static void MapImplicitAreaControllerRoute(this IEndpointRouteBuilder rou
         defaults: new { controller = areaName }
       );
 
+    /// <summary>
+    ///   Adds the <c>{area:exists}/{action=Index}</c> endpoint route for all areas where the controller has the same name as
+    ///   the area.
+    /// </summary>
+    /// <remarks>
+    ///   <para>
+    ///     This extension method implicitly assigns the controller name based on the area name. This is advantageous when there
+    ///     are multiple areas which have a single controller which is named after the area—e.g., <c>[Area("Forms")]</c> and
+    ///     <c>FormsController: Controller</c>—as this allows those to be collectively registered under a single route, without
+    ///     needing the redundant <c>Controller</c> value to be defined in the route (e.g., <c>/Forms/Forms/{action}</c>.
+    ///   </para>
+    ///   <para>
+    ///     Be aware that this method uses the <see cref="ControllerEndpointRouteBuilderExtensions.MapDynamicControllerRoute{
+    ///     TTransformer}(IEndpointRouteBuilder, String)"/> method. In .NET 3.x, this is incompatible with both the <see cref=
+    ///     "AnchorTagHelper"/> and <see cref="LinkGenerator"/> classes. This means that e.g. <c>@Url.Action()</c> references
+    ///     in views won't be properly formed. If these are required, prefer registering each route individually using <see
+    ///     cref="MapImplicitAreaControllerRoute(IEndpointRouteBuilder, String)"/>.
+    ///   </para>
+    /// </remarks>
+    public static void MapImplicitAreaControllerRoute(this IEndpointRouteBuilder routes) =>
+      routes.MapDynamicControllerRoute<TopicRouteValueTransformer>("{area:exists}/{action=Index}");
+
     /*==========================================================================================================================
     | EXTENSION: MAP TOPIC REDIRECT (IENDPOINTROUTEBUILDER)
     \-------------------------------------------------------------------------------------------------------------------------*/