Add definition and test file for StuffIt X archives.

Mark Ultra requested to merge okeeblow/shared-mime-info:stuffitx into master

There is an existing definition for the older StuffIt and StuffIt 5.5 formats. This newer format was added in StuffIt 7 in 2002 with separate Media-Type.

From StuffIt Deluxe on my PowerBook 2400c:

  <string>StuffIt X Archive</string>

My included test.sitx was created with the same version of the software on the same machine:

[okeeblow@emi#shared-mime-info] xxd -l 16 tests/mime-detection/test.sitx
00000000: 5374 7566 6649 7421 a10a 4a05 9bdd 4215  StuffIt!..J...B.

Apache HTTPd is aware of this Media-Type:

PRONOM format definition:

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