feat: Adds vertical and horizontal separators, renames modifier protocol.

Sources/CliDocCore/TextModifier.swift

@@ -0,0 +1,11 @@
+public protocol TextModifier {
+  // swiftlint:disable type_name
+  associatedtype _Body: TextNode
+  typealias Body = _Body
+  // swiftlint:enable type_name
+
+  associatedtype Content
+
+  @TextBuilder
+  func render(content: Content) -> Body
+}