feat: Moves core functionality into it's own library.

Sources/CliDocCore/NodeModifier.swift

@@ -0,0 +1,48 @@
+import Rainbow
+
+public protocol NodeModifier {
+  // swiftlint:disable type_name
+  associatedtype _Body: TextNode
+  typealias Body = _Body
+  // swiftlint:enable type_name
+
+  associatedtype Content
+
+  @TextBuilder
+  func render(content: Content) -> Body
+}
+
+public struct ModifiedNode<Content: TextNode, Modifier: NodeModifier> {
+
+  @usableFromInline
+  let content: Content
+
+  @usableFromInline
+  let modifier: Modifier
+
+  @usableFromInline
+  init(content: Content, modifier: Modifier) {
+    self.content = content
+    self.modifier = modifier
+  }
+}
+
+extension ModifiedNode: TextNode where Modifier.Content == Content {
+  public var body: some TextNode {
+    modifier.render(content: content)
+  }
+}
+
+extension ModifiedNode: NodeRepresentable where Self: TextNode {
+  @inlinable
+  public func render() -> String {
+    body.render()
+  }
+}
+
+public extension TextNode {
+  @inlinable
+  func modifier<M: NodeModifier>(_ modifier: M) -> ModifiedNode<Self, M> {
+    .init(content: self, modifier: modifier)
+  }
+}