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.