Add support for zstd compression

[COUR4G3]

Encountered a tilemap with zstd compression, easy enough to load and save it without but thought since there was a module on pip for zstd, might just add some optional support - if zstd is installed, then support decompression.

Something like:

try:
    import zstd
except ImportError:
    zstd = False

# ... in decompression code

if compression == "zstd":
    if zstd:
        data = zstd.decompress(data)
    else:
        raise ValueError("layer compression zstd is not supported. Install zstd to enable support.")

Already have a PR drafted if there is interest.