Function type declaration for UText.copy().
Copy or move a substring from one position to another within the text, while retaining any metadata associated with the text. This function is used to duplicate or reorder substrings. The destination index must not overlap the source range.
The text to be copied or moved is inserted at destIndex; it does not replace or overwrite any existing text.
This function need only be implemented for UText types that support writing.
When using this function, there should be only a single UText opened onto the underlying native text string. The function is responsible for updating the text chunk within the UText to reflect the updated iteration position, taking into account any changes to the underlying string's structure caused by the replace operation.