Add zstd to prcompression values

[yetzt]

zstd is supported by chrome now and will probably be supported by other browsers in the near future.

[MichaelKreil]

makes sense!
little test using maximum compression on vector tile 0/0/0:

compression	size
uncompressed	436'324
gzip	116'387
zstd	99'028
brotli	92'685

But we might want to wait for broader support: https://caniuse.com/zstd

[yetzt]

i think the advantage is faster compression and decompression at comparable rates, which would be advantageous when creating or recompressing tilesets. for use of a versatiles container as a source in a tileserver it already makes sense.