Add @supports at-rule(@keyword)

Details in https://www.bram.us/2026/03/15/at-rule/ Status: Chromium (Blink) - ✅ Supported in Chromium 148 and up Firefox (Gecko) - ❌ No support, [Issue #1751188](https://bugzilla.mozilla.org/show_bug.cgi?id=1751188) Safari (WebKit) -❌ No support, [Issue #235400](https://bugs.webkit.org/show_bug.cgi?id=235400)