Add machine-readable empty-column indices to csvclean

[Rober1208]

csvclean --empty-columns reports the affected column numbers only inside the
human-readable msg field. Scripts that need those numbers currently have to
parse that message.

Add --empty-column-indices, which enables the empty-column check and adds an
empty_column_indices field to the error CSV on standard error. It contains a
comma-separated list such as 2,4,5, or 1,3,4 with --zero; it is blank for
other error rows. The existing error CSV format stays the same unless this
option is used. Documentation, the man page, and the changelog are updated.

Refs #1290.

Validation:

• The new regression failed on the unchanged base because the option did not
  exist, and passed after the change.
• PYTHONUTF8=1 python -m pytest -q: 357 passed, 31 subtests passed.
• flake8 csvkit tests, isort csvkit tests --check-only, and
  check-manifest: passed.
• A subprocess check confirmed that csvcut -c empty_column_indices reads
  2,4,5 from the new error CSV.

This contribution was implemented and verified by Codex AI on behalf of
Rober1208. It has not had an independent human code review.

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