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.Models/StringDocument.cs

@@ -0,0 +1,15 @@
+using System.Text;
+
+namespace Beam.Models {
+    public class StringDocument(string filename, string content, Encoding? encoding = null) : Document(filename, encoding) {
+        public string Content { get; set; } = content;
+
+        public override byte[] ToBytes() {
+            return Encoding.GetBytes(Content);
+        }
+
+        public override string ToString() {
+            return Content;
+        }
+    }
+}