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

Sources/CliDocCore/Nodes/Group.swift

@@ -0,0 +1,16 @@
+public struct Group<Content: TextNode>: TextNode {
+  @usableFromInline
+  var content: Content
+
+  @inlinable
+  public init(
+    @TextBuilder content: () -> Content
+  ) {
+    self.content = content()
+  }
+
+  @inlinable
+  public var body: some TextNode {
+    content
+  }
+}