Add gawk-compatible CSV mode (`--csv`)

[bertysentry]

Requirements

The current gawk compatibility harness records --csv as unsupported and skips the corresponding gawk fixtures.

Current coverage gap:

• src/it/java/io/jawk/GawkMaketestsParser.java treats --csv as an unsupported flag today.
• src/it/resources/gawk/skips.properties currently skips csv1, csv2, csv3, and csvodd with the reason gawk's --csv mode is not implemented by Jawk.
• The vendored gawk manifest contains the relevant rules in src/it/resources/gawk/Maketests.

We should add a gawk-compatible CSV mode to Jawk's CLI and runtime so these tests can run instead of being skipped.

Acceptance criteria:

• Jawk accepts --csv on the CLI.
• Record splitting and field parsing are compatible enough with gawk's CSV mode for the vendored csv1, csv2, csv3, and csvodd fixtures.
• The compatibility suite can unskip those cases and compare their real outputs.
• The behavior is covered by tests using AwkTestSupport and/or the existing gawk IT harness.

Suggestions

Implement CSV support with a dedicated CsvInputSource class that implements InputSource.