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/IStateChangeBehaviour.cs

@@ -0,0 +1,10 @@
+using Beam.Models;
+
+namespace Beam.Abstractions;
+
+/// <summary>
+/// Defines how a url template should should be updated, in what order, and by how much
+/// </summary>
+public interface IStateChangeBehaviour {
+    public void Apply(IState state, object stimulus);
+}