.tab extension is a known extension for tab-separated values
according to Wikipedia.

Also add .tsv .tab files to the list of known text files, so XML
detection code won't run on it, and won't output errors to console.

Change-Id: Ifb8edc40900cb4669264e1e989c01efd61ff24bf
Reviewed-on: https://gerrit.libreoffice.org/7038Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>