Closes #19891

Similar to VSCode’s `editor.roundedSelection` option.

#### Before/after

<table>
<tr><th><th>Enabled (default)</th><th>Disabled</th>
<tr><td>Editor-based UIs<td><img width="268" height="58" alt="image"
src="https://github.com/user-attachments/assets/f58c6817-88fc-4cba-b2bc-f7eff58ec6e5"
/>
<img width="146" height="97" alt="image"
src="https://github.com/user-attachments/assets/0cd08afa-8243-4d4e-a5c6-9055f6834ecf"
/><td><img width="272" height="54" alt="image"
src="https://github.com/user-attachments/assets/286c8f53-1973-442e-8446-4f48e3feca30"
/>
<img width="133" height="90" alt="image"
src="https://github.com/user-attachments/assets/4aea2044-403c-47a5-bb6d-a88a0b65814e"
/></td>
<tr><td>Terminal<td><img width="287" height="84" alt="image"
src="https://github.com/user-attachments/assets/b1594f68-2ef6-4bdc-9030-e67d55a5bf99"
/><td><img width="289" height="79" alt="image"
src="https://github.com/user-attachments/assets/6d095d9d-b408-4440-a9f5-6a2af2b84b61"
/></td>
</table>

Release Notes:

- Added setting `rounded_selection` to disable rounded corners for text
selection.