feat: Working on node builder

Sources/CliDoc/ArgumentParserExtensions/Abstract.swift

@@ -0,0 +1,16 @@
+/// Use the `NodeBuilder` for generating an abstract.
+public struct Abstract: NodeRepresentable {
+  @usableFromInline
+  let label: any NodeRepresentable
+
+  @inlinable
+  public init(@NodeBuilder label: () -> any NodeRepresentable) {
+    self.label = label()
+  }
+
+  @inlinable
+  public func render() -> String {
+    label.render()
+  }
+
+}