Auto-detect CSV format when `--csv-delimiter` is specified

[dennisangemi]

Problem Description

Currently, when using --csv-delimiter with a non-default delimiter (e.g., semicolon), VisiData doesn't automatically recognize the file as CSV format. This results in:

1. "unknown filetype" message for files without .csv extension
2. File opened as plain text instead of CSV
3. User must manually specify -f csv in addition to --csv-delimiter

Current Behavior

# This shows "unknown filetype" and opens as text
echo "Name;Age;City
John;25;New York
Alice;30;London" | vd --csv-delimiter ";" -

# User must specify both flags
echo "Name;Age;City
John;25;New York  
Alice;30;London" | vd --csv-delimiter ";" -f csv -

Expected Behavior

When --csv-delimiter is specified with a non-default value, VisiData should automatically assume CSV format without requiring the -f csv flag.

Use Case

This is particularly useful for:

• CSV files using semicolons as delimiters
• Data files without .csv extension but with CSV structure
• Simplified command-line usage when working with non-comma CSV data

Environment

VisiData version: 3.2+
Files affected: Any text file with CSV structure using non-comma delimiters

Pull request

I'm opening a pull request to implement this feature