copy_and_paste: Paste text first unformatted and then formatted.

This commit makes pasting formatted text a two-step process:

- Insert unformatted text
- Insert formatting

This way, undo (Ctrl+Z) will restore the original pre-formatting
syntax.

This is the same as the approach taken with
auto-formatting in #29302.

Fixes #31061

web/src/copy_and_paste.ts

@@ -745,6 +745,11 @@ export function paste_handler(this: HTMLTextAreaElement, event: JQuery.Triggered
             event.stopPropagation();
             paste_html = maybe_transform_html(paste_html, paste_text);
             const text = paste_handler_converter(paste_html);
+            // Pasting formatted text is a two-step process:
+            // First we paste unformatted text,
+            // then overwrite it with formatted text.
+            // This way, undo restores the pre-formatting syntax.
+            add_text_and_select(trimmed_paste_text, $textarea);
             compose_ui.insert_and_scroll_into_view(text, $textarea);
         }
     }