refactor: unify binding & data provider interfaces

- Removed BindingType enum and all related logic from Binding.
- Made Binding implement new IBinding and IKeyed interfaces.
- Moved node selection logic to IBinding.Select; removed Resolve* methods from Binding.
- Added new IBinding interface for XPath/CssPath selection.
- Refactored IDataProvider to generic IDataProvider<T>; removed GetNode.
- Updated ListContentDataProvider and ParagraphedContentDataProvider to use IBinding.
- Added new ContentsDataProvider, ContentsArrayDataProvider, and DropDownDataProvider for flexible data extraction.
- Updated DataBindings to use IDataProvider<T> properties instead of Binding.
- Updated all usages to new interfaces and patterns.

Beam.Dynamic/ContentsDataProvider.cs

@@ -0,0 +1,20 @@
+using HtmlAgilityPack;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Beam.Dynamic {
+    public class ContentsDataProvider : IDataProvider<string> {
+        public IBinding? Content { get; set; }
+
+        public string Get(HtmlDocument document) {
+            if (Content is null)
+                return "";
+
+            return Content.Select(document)?.InnerText ?? "";
+        }
+    }
+}