Improve textarea paste (part of gitea#31948)

When pasting a URL over another URL, replace the URL instead of creating
a useless `[url](url)`.

(Partially picked from commit 55502265716f000a2c61c98fa093244e1310d97f)

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

web_src/js/features/comp/Paste.js

@@ -116,7 +116,7 @@ function handleClipboardText(textarea, text, e) {
   if (_shiftDown) return;
   const selectedText = value.substring(selectionStart, selectionEnd);
   const trimmedText = text.trim();
-  if (selectedText && isUrl(trimmedText)) {
+  if (selectedText && isUrl(trimmedText) && !isUrl(selectedText)) {
     e.stopPropagation();
     e.preventDefault();
     replaceTextareaSelection(textarea, `[${selectedText}](${trimmedText})`);