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.Exceptions/AssertionException.cs

@@ -0,0 +1,14 @@
+namespace Beam.Exceptions;
+
+/// <summary>
+/// The kind of exception that should never happen
+/// </summary>
+[Serializable]
+public class AssertionException : Exception {
+    public AssertionException() { }
+    public AssertionException(string message) : base(message) { }
+    public AssertionException(string message, Exception inner) : base(message, inner) { }
+    protected AssertionException(
+        System.Runtime.Serialization.SerializationInfo info,
+        System.Runtime.Serialization.StreamingContext context) : base(info, context) { }
+}