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/Document.cs

@@ -0,0 +1,16 @@
+using System.Text;
+using aeqw89.DataKeys;
+using Beam.Abstractions;
+
+namespace Beam.Models {
+    public abstract class Document(string filename, Encoding? encoding = null) : IDocument {
+        public string Filename { get; set; } = filename;
+        public Encoding Encoding { get; set; } = encoding ?? Encoding.UTF8;
+        public Dictionary<DataKey<IDocumentMetaData>, IDocumentMetaData> MetaData { get; set; } = [];
+        IDictionary<IDataKey<IDocumentMetaData>, IDocumentMetaData> IDocument.MetaData 
+            => MetaData.ToDictionary(IDataKey<IDocumentMetaData> (x) => x.Key, x => x.Value);
+
+        public abstract byte[] ToBytes();
+        public abstract override string ToString();
+    }
+}