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/State.cs

@@ -0,0 +1,23 @@
+using Beam.Abstractions;
+
+namespace Beam.Models {
+    public class State(string[] state) : IState {
+        string[] state = state;
+
+        public string[] GetState() => state;
+        public void SetState(string[] state) => this.state = state;
+
+        public State Copy()
+            => new((string[])state.Clone());
+        
+        IReadOnlyState IReadOnlyState.Copy() 
+            => Copy();
+        IState IState.Copy() 
+            => Copy();
+
+        public string this[Index i] {
+            get => state[i];
+            set => state[i] = value;
+        } 
+    }
+}