feat: Working on node builder

Sources/CliDoc/NodeModifier.swift

@@ -0,0 +1,9 @@
+public protocol NodeModifier: Sendable {
+  func render(_ node: any NodeRepresentable) -> any NodeRepresentable
+}
+
+public extension NodeRepresentable {
+  func modifier(_ modifier: NodeModifier) -> any NodeRepresentable {
+    modifier.render(self)
+  }
+}