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/ILinkBuilder.cs

@@ -0,0 +1,11 @@
+using Beam.Models;
+
+namespace Beam.Abstractions;
+
+public interface ILinkBuilder {
+    /// <summary>
+    /// Produces a concrete <see cref="SourceLink"/> using values from an external <see cref="State"/> object.
+    /// </summary>
+    /// <param name="parameterValues">Object providing positional values.</param>
+    string Build(IReadOnlyState parameterValues);
+}