feat: zstandard support

[Phillip9587]

This PR introduces support for the Zstandard (Zstd) algorithm, which was released as an experimental feature in Node.js 23.8.0.

Key Changes

• Adds support for Zstandard (Zstd) as an optional compression algorithm.

Notes

• This feature is experimental and subject to changes in future Node.js releases.
• Requires Node.js 23.8.0+ to function correctly.

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closes #560

[bjohansebas]

I would really prefer to wait until the API becomes stable. This is the same stance I’m going to take with compression. There may be no changes during the transition from experimental to stable, but it’s better to use only stable APIs

Great job anyway!

[wesleytodd]

I agree, we should wait until it is stable then just enable support without an experimental phase.

[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-zlibzstddecompress

[Phillip9587]

As @bjohansebas mentioned in expressjs/compression#250 (comment), several ecosystem packages have already adopted zstd. Should we revisit this discussion? Zstd was backported to Node.js v22.15.0, so once Node.js 20 LTS reaches EOL in April, all active LTS lines will include zstd support.