rewrite local media scanner (#25)

* spike new scanner

* add existing items to new scanner

* add collection refresh actions

* add tv show metadata and posters

* update metadata and posters when nfo/poster files are updated

* add "remove" action, test for all supported file extensions

* update statistics when primary video file is updated

* reflect that collections are "sourced" from nfo

* implement most scanning actions

* cleanup

* fix startup

* cross-platform scanner tests

ErsatzTV.Core/Metadata/LocalMediaSourcePlan.cs

@@ -0,0 +1,11 @@
+using System.Collections.Generic;
+using ErsatzTV.Core.Domain;
+using LanguageExt;
+
+namespace ErsatzTV.Core.Metadata
+{
+    public record LocalMediaSourcePlan(Either<string, MediaItem> Source, List<ActionPlan> ActionPlans)
+    {
+        public Either<string, MediaItem> Source { get; set; } = Source;
+    }
+}