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/StateChangerFactory.cs

@@ -4,9 +4,10 @@ using System.Linq;
 using System.Text;
 using System.Text.Json.Serialization;
 using System.Threading.Tasks;
+using Beam.Abstractions;
 
 namespace Beam.Dynamic {
-    public class StateChangerFactory {
+    public class StateChangerFactory : IStateChangerFactory {
         [JsonIgnore]
         public IStateChangeBehaviour Behavior => FactoryTable[StateChangerKey]();