Add a default mapping for the .tsv extension to point to okf_table_tsv

Even though TSV is a variant of CSV, some people use the .tsv suffix specifically for TSVs, to distinguish them from CSVs. (For example, .tsv is listed as an affected for the file type [in Wikipedia(https://en.wikipedia.org/wiki/Tab-separated_values)).

Currently, we don't register a mapping for .tsv to the okf_table_tsv filter in Rainbow etc. We should, since it wouldn't conflict with anything.