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

Sources/CliDocCore/Nodes/Empty.swift

@@ -0,0 +1,11 @@
+/// An empty text node.
+public struct Empty: TextNode {
+
+  @inlinable
+  public init() {}
+
+  @inlinable
+  public var body: some TextNode {
+    ""
+  }
+}