mime-types: Add application/zstd to supported mime-types

Currently, it isn't possible to use Nautilus for the extraction of the ".zstd" and ".tar.zst" archives. However, it seems that this compression is supported by the libarchive library nowadays. Let's add the application/zstd and application/x-zstd-compressed-tar mime-types to the list of supported. This will allow Nautilus to show the "Extract" and "Extract to..." actions.

Related: nautilus#2860 (closed)