add `text/x-tex` to type document

This adds the mime type text/x-tex to the type "Document".

.tex is a typesetting system for scientific documents, and the de facto standard file type for text documents in academia, of comparable nature (and for users who work with it, relevance) as office word or markdown documents. It should therefore appear in search results of type "Document".

Related threads:
https://www.reddit.com/r/kde/comments/tlt6bv/why_baloo_does_not_provide_all_its_results_to/
https://askubuntu.com/questions/1224569/krunner-wont-find-certain-files
https://www.reddit.com/r/kde/comments/mp4w87/krunner_wont_find_certain_files/