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.Exports/Exporter.cs

@@ -1,8 +1,11 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Net;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Abstractions;
+using Beam.Models;
 
 namespace Beam.Exports {
     public class PlainTextExporter : IExporter, IAsyncExporter {
@@ -24,14 +27,14 @@ namespace Beam.Exports {
             var text = Convert();
             if (!Directory.Exists(Path.GetDirectoryName(path)))
                 throw new ArgumentException(S.M.FileDirectoryDoesNotExist, nameof(path));
-            File.WriteAllText(path, text, Encoding.Unicode);
+            System.IO.File.WriteAllText(path, text, Encoding.Unicode);
         }
 
         public virtual async Task WriteAsync(string path) {
             var text = await ConvertAsync();
             if (!Directory.Exists(path))
                 throw new ArgumentException(S.M.FileDirectoryDoesNotExist, nameof(path));
-            await File.WriteAllTextAsync(path, text);
+            await System.IO.File.WriteAllTextAsync(path, text);
         }
     }
 }