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/ListContentDataProvider.cs

@@ -2,14 +2,14 @@
 using System.Text;
 
 namespace Beam.Dynamic {
-    public class ListContentDataProvider : IDataProvider {
-        public Binding? Content { get; set; }
+    public class ListContentDataProvider : IDataProvider<string> {
+        public IBinding? Content { get; set; }
 
         public string Get(HtmlDocument document) {
             if (Content is null)
                 return "";
 
-            var node = Content.ResolveNode(document);
+            var node = Content.Select(document);
             if (node is null)
                 return "";
 
@@ -23,9 +23,5 @@ namespace Beam.Dynamic {
             content.Append(node.ChildNodes.Last().InnerText.Trim());
             return content.ToString();
         }
-
-        public HtmlNode? GetNode(HtmlDocument document) {
-            return Content?.ResolveNode(document);
-        }
     }
 }