Support TSV (Tab-Separated Values) files

[anandmt]

Description

The CsvConverter currently only handles .csv files with comma delimiters. TSV (Tab-Separated Values) files are extremely common in data science, bioinformatics, spreadsheet exports, and LLM pipelines, but are not supported.

Expected behavior

MarkItDown should convert .tsv files to Markdown tables, just like it does for .csv files.

Proposed solution

Extend the existing CsvConverter to:

1. Accept .tsv files and text/tab-separated-values MIME type
2. Auto-detect the delimiter using Python's built-in csv.Sniffer
3. Fall back to tab for .tsv files, comma for .csv files

PR

#2021