[KFileMetaData] Add extractor for DSC conforming (Encapsulated) Postscript

Summary:
Postscript files currently fall back to the plaintext extractor due to
mimetype inheritance. This adds lots of garbage to the index and misses
any useful data in the DSC comments.

Test Plan: make && ctest

Reviewers: Frameworks, Baloo, astippich, ngraham, poboiko

Reviewed By: poboiko

Subscribers: pino, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16498