refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Dynamic/AnchorDataProvider.cs

@@ -6,7 +6,7 @@ using System.Text;
 using System.Threading.Tasks;
 
 namespace Beam.Dynamic {
-    public class AnchorDataProvider : IDataProvider<SourceLink>, IDataProvider<string> {
+    public class AnchorDataProvider : IDataProvider<string> {
         public IBinding? Content { get; set; }
 
         public string Get(HtmlDocument document) {
@@ -16,16 +16,5 @@ namespace Beam.Dynamic {
             return Content.Select(document)?.GetAttributeValue("href", "") ?? "";
 
         }
-
-        SourceLink IDataProvider<SourceLink>.Get(HtmlDocument document) {
-            var content = Get(document);
-            if (content is null)
-                return SourceLink.InvalidLink;
-
-            if (!Uri.TryCreate(content, UriKind.RelativeOrAbsolute, out _))
-                return SourceLink.InvalidLink;
-
-            return new SourceLink(content);
-        }
     }
 }