Make TextView non-final

Sources/Runestone/TextView/TextView.swift

@@ -11,7 +11,7 @@ import UIKit
 ///
 /// When initially configuring the `TextView` with a theme, a language and the text to be shown, it is recommended to use the ``setState(_:addUndoAction:)`` function.
 /// The function takes an instance of ``TextViewState`` as input which can be created on a background queue to avoid blocking the main queue while doing the initial parse of a text.
-open final class TextView: UIScrollView {
+open class TextView: UIScrollView {
     /// Delegate to receive callbacks for events triggered by the editor.
     public weak var editorDelegate: TextViewDelegate?
     /// Whether the text view is in a state where the contents can be edited.