Change zstd extension names

[defrag257]

Change zstd extension names (for 7zFM archive creation dialog and 7z console edition) to zst tzst.

[mcmilk]

I think some support for .zstd should be left ... so up2now generated archives can be read without hassle.

[defrag257]

I think some support for .zstd should be left ... so up2now generated archives can be read without hassle.

Extraction uses magic numbers to detect file type... so this change doesn't affect it.
It affects existing scripts using 7z.exe to create an archive without specifying -tzstd:

7z a abc.tar.zstd abc.tar        # does not work after this change
7z a -tzstd abc.tar.zstd abc.tar # still works

If this compatibility is necessary, I think the code could be changed to:

REGISTER_ARC_IO(
  "zstd", "zst zstd tzst tzstd", "* * .tar .tar", 0x0e,
  k_Signature,
  0,
  NArcInfoFlags::kKeepName,
  0,
  IsArc_zstd)

[defrag257]

GzHandler.cpp uses the same pattern, so it should work.

[mcmilk]

GzHandler.cpp uses the same pattern, so it should work.

I also think this should work. I will squash the commits together and apply them later.
Thanks a lot.

[mcmilk]

I rebased the commits and merged it.