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

Sources/CliDocCore/TextNode.swift

@@ -0,0 +1,67 @@
+public protocol NodeRepresentable {
+  func render() -> String
+}
+
+public protocol TextNode: NodeRepresentable {
+  // swiftlint:disable type_name
+  associatedtype _Body: TextNode
+  typealias Body = _Body
+  // swiftlint:enable type_name
+
+  var body: Body { get }
+}
+
+public extension TextNode {
+  func render() -> String {
+    body.render()
+  }
+}
+
+// MARK: - String
+
+extension String: NodeRepresentable {
+  public func render() -> String {
+    self
+  }
+}
+
+extension String: TextNode {
+  public var body: some TextNode {
+    self
+  }
+}
+
+// MARK: - Optional
+
+extension Optional: TextNode where Wrapped: TextNode {
+  @TextBuilder
+  public var body: some TextNode {
+    switch self {
+    case let .some(node):
+      node
+    case .none:
+      Empty()
+    }
+  }
+}
+
+extension Optional: NodeRepresentable where Wrapped: NodeRepresentable, Wrapped: TextNode {
+
+  public func render() -> String {
+    body.render()
+  }
+}
+
+// MARK: - Array
+
+extension Array: TextNode where Element: TextNode {
+  public var body: some TextNode {
+    NodeContainer(nodes: self)
+  }
+}
+
+extension Array: NodeRepresentable where Element: NodeRepresentable, Element: TextNode {
+  public func render() -> String {
+    body.render()
+  }
+}