Add configurable and auto-detected CSV separators

[enijburg]

Motivation

• Allow CSV mode to handle files that use different delimiters and let users override or let the viewer pick the best delimiter automatically.
• Improve CSV rendering alignment for non-comma-separated files and support \t escape for tab separators.

Description

• Add an optional CLI flag --csv-separator (Args::csv_separator) to accept a single ASCII separator or \t and thread it into Viewer::open as Option<&str>.
• Implement resolve_csv_separator, parse_csv_separator and detect_csv_separator to parse a user-provided separator or auto-detect among candidates [',',';','\t','|',':'] by sampling lines.
• Store the chosen separator in Viewer as csv_separator: Option<u8>, change index_csv_column_widths to accept a separator: u8, and update rendering in render_line to use the configured/detected separator for alignment and display.
• Add unit tests for separator detection and parsing and update existing CSV-related test helpers and call sites to the new function signatures.

Testing

• Ran code formatting with cargo fmt (succeeded).
• Ran the test suite with cargo test and all tests passed (20 passed; 0 failed).

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Codex Task