Create public API endpoint for copy()

Harlequin has to do this to copy data from the table: ```py @on(DataTable.SelectionCopied) def copy_data_to_clipboard(self, message: DataTable.SelectionCopied) -> None: message.stop() # Excel, sheets, and Snowsight all use a TSV format for copying tabular data text = os.linesep.join("\t".join(map(str, row)) for row in message.values) self.editor.text_input.clipboard = text if self.editor.use_system_clipboard: try: self.editor.text_input.system_copy(text) except Exception: self.notify("Error copying data to system clipboard.", severity="error") else: self.notify("Selected data copied to clipboard.") ```