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.Stealth/Beam.Stealth.csproj

@@ -10,8 +10,6 @@
         <PackageReference Include="Selenium.WebDriver" Version="4.34.0" />
     </ItemGroup>
     <ItemGroup>
-        <ProjectReference Include="..\Beam\Beam.csproj">
-            <PrivateAssets>all</PrivateAssets>
-        </ProjectReference>
+      <ProjectReference Include="..\Beam.Downloaders\Beam.Downloaders.csproj" />
     </ItemGroup>
 </Project>

Beam.Stealth/StealthFragmentDownloader.cs

@@ -4,6 +4,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Downloaders;
+using Beam.Models;
 
 namespace Beam.Stealth {
     public class StealthFragmentDownloader<T> : UnitFragmentDownloaderBinary<T> {

Beam.Stealth/StealthFragmentPageDownloader.cs

@@ -5,6 +5,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Downloaders;
+using Beam.Models;
 
 namespace Beam.Stealth {
     public class StealthFragmentPageDownloader<T> : UnitFragmentDownloader<T> {

Beam.Stealth/StealthUnitDownloader.cs

@@ -6,8 +6,12 @@ using System.Diagnostics;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Downloaders;
+using Beam.Models;
 
 namespace Beam.Stealth {
+    using File = System.IO.File;
+    
     public class StealthUnitDownloader<T> : UnitDownloaderBinary<T> {
         public StealthConfig Config { get; }
         public StealthAsyncManipulator Manipulator { get; }

Beam.Stealth/StealthUnitPageDownloader.cs

@@ -5,6 +5,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Downloaders;
+using Beam.Models;
 
 namespace Beam.Stealth {
     public class StealthUnitPageDownloader<T> : UnitDownloader<T> {