Support zstd decoding

[ikreymer]

Browsers now include Accept-Encoding: zstd, and more and more sites are serving zstd compression, so should definitely add support for zstd decompression, along with deflate/gzip/brotli that wabac.js supports.

[ikreymer]

Priority is for zstd encoded content first, but would also be great to support zstd compressed WARCs which are also in use.

Will look at https://github.com/101arrowz/fzstd first, also considering WASM options.