Add support for zstd

[polys]

Adds support for zstd, but keeps brotli as preferred.

Resolves #217

[Phillip9587]

I think we decided to wait for zstd to become stable in Node.js before using it:

• feat: zstandard support body-parser#586
• zstd support? body-parser#560

[polys]

I think we decided to wait for zstd to become stable in Node.js before using it:

Thank you, that's perfectly fine :)
I did the work for a little experiment of mine and thought I'd open the PR in case it's useful when the time is right.

[DienoX]

Isn't it that time already?

[Phillip9587]

Zsdt is still experimental as of Node.js 25.6.1: https://nodejs.org/api/zlib.html#class-zlibzstdcompress

[bjohansebas]

While it’s still experimental, it’s already being adopted by several important packages like fastify/compress, webpack-bundle-analyzer, and likely many others. I think it would be a good idea to include it. There haven’t been many issues reported about this, and the existing ones seem more like Node.js-related problems rather than ours.

That said, I do wonder whether this should probably be a major version or not. I’m asking because when I implemented Brotli in compression as a minor release, issues were reported by people who couldn’t control which encodings were accepted.