Add zstd compression support

[terrelln]

• Zstd support has been added to SquashFS in the kernel in commit https://github.com/torvalds/linux/commit/87bf54bb43ddd385d2538b777324bf737f243042.
• Support is ready for mksquashfs and is coming to squashfs-tools soon https://sourceforge.net/p/squashfs/mailman/message/35996905/.

[terrelln]

CC @iburinoc @chipturner @Cyan4973

[probonopd]

Thank you @terrelln @chipturner