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.Abstractions/IDocument.cs

@@ -0,0 +1,28 @@
+using aeqw89.DataKeys;
+
+namespace Beam.Abstractions;
+
+public interface IDocument {
+    /// <summary>
+    /// The file name of the document. Must be valid in both <c>UNIX</c>, 
+    /// <c>WINDOWS</c>, <c>APPLE</c>, and <c>ANDROID</c> file systems.
+    /// </summary>
+    string Filename { get; }
+
+    /// <summary>
+    /// Additional descriptive data
+    /// </summary>
+    IDictionary<IDataKey<IDocumentMetaData>, IDocumentMetaData> MetaData { get; }
+
+    /// <summary>
+    /// Retrieves the binary representation for the <see cref="IDocument"/>
+    /// </summary>
+    /// <returns>Binary representation of the <see cref="IDocument"/></returns>
+    byte[] ToBytes();
+
+    /// <summary>
+    /// Retrieves the string representation for the <see cref="IDocument"/>
+    /// </summary>
+    /// <returns>String representation of the <see cref="IDocument"/></returns>
+    string ToString();
+}