support zstd for send-compressed

[Jxck]

currently, file.send-compressed: ON only supports "gzip" and "br".
and now some browsers working on support "zstd" in content-encoding.
I'm now planning deploy pre-compiled ".zstd" file on server so it's welcome to include zstd in send-compressed.

• (Intent to Ship: Zstd Content-Encoding](https://groups.google.com/a/chromium.org/g/blink-dev/c/GFFOLCF12a4/m/GuYQ4LRQAgAJ)
• Gecko: Positive (mozilla/standards-positions#775)
• WebKit: Positive (WebKit/standards-positions#168)