fix textarea newline handle (#32966)

- Fix cursor position if input newline on middle of lines
- ~Increment number if numbered list~


![image](https://github.com/user-attachments/assets/bcfe2625-11a8-4ea4-9a71-b7ecfe81b2e0)

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/comp/EditorMarkdown.ts

@@ -92,6 +92,7 @@ function handleNewline(textarea: HTMLTextAreaElement, e: Event) {
   if (!line) {
     // clear current line if we only have i.e. '1. ' and the user presses enter again to finish creating a list
     textarea.value = value.slice(0, lineStart) + value.slice(lineEnd);
+    textarea.setSelectionRange(selStart - prefix.length, selStart - prefix.length);
   } else {
     // start a new line with the same indention and prefix
     let newPrefix = prefix;