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.Tests/OnlineCleaner.Tests.cs

@@ -8,12 +8,12 @@ using System.Text;
 using System.Threading.Tasks;
 
 namespace Beam.Tests {
-    public class OnlineCleanerTests {
+    public class StringCleanerTests {
         [Fact]
         public void Should_LocalizeArabic() {
             const string test = "1234";
 
-            List<int> localized = OnlineCleaner.ParseNumbers(test,  Culture.English);
+            List<int> localized = StringCleaner.ParseNumbers(test,  Culture.English);
 
             Assert.Single(localized);
             Assert.Equal(1234, localized[0]);
@@ -23,7 +23,7 @@ namespace Beam.Tests {
         public void Should_LocalizeIndian() {
             const string test = "九一五";
 
-            List<int> localized = OnlineCleaner.ParseNumbers(test, Culture.Chinese);
+            List<int> localized = StringCleaner.ParseNumbers(test, Culture.Chinese);
 
             Assert.Single(localized);
             Assert.Equal(915, localized[0]);