Add @supports at-rule

[Schweinepriester]

https://www.bram.us/2024/07/11/feature-detect-css-starting-style-support/:

While in theory you could use @supports at-rule() for this, in practice you can't because it has no browser support (😭).

https://www.bram.us/2022/01/20/detect-at-rule-support-with-the-at-rule-function/:

the CSS Working Group just decided to introduce a new function that be used in conjunction with @supports: at-rule().

With it, we can feature detect at-rule support as follows:

@supports at-rule(@foo) {
  /* @foo is supported */
}

• WebKit/Safari: Issue #235400
• Gecko/Firefox: Issue #1751188
• Blink/Chromium: Issue #1289224

I feel like it's very likely that this will come via MDN, but as usual just keeping track 🤓