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

Sources/CliDocCore/Nodes/AnyTextNode.swift

@@ -0,0 +1,21 @@
+/// A type-erased text node.
+public struct AnyTextNode: TextNode {
+  @usableFromInline
+  let makeString: () -> String
+
+  @inlinable
+  public init<N: TextNode>(_ node: N) {
+    self.makeString = node.render
+  }
+
+  @inlinable
+  public var body: some TextNode { makeString() }
+}
+
+public extension TextNode {
+
+  @inlinable
+  func eraseToAnyTextNode() -> AnyTextNode {
+    AnyTextNode(self)
+  }
+}