grep: let dot match newline in null-data mode

[wondr-wclabs]

Closes #25.

Summary

• enable Onig's dot-matches-newline option when --null-data is active
• keep the change scoped to NUL-delimited records, where newline is ordinary data rather than the record separator
• add a byte-level -z -o . regression for a record containing a\nb

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 69 / FAILED: 38 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 70 / FAILED: 37 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: -1
  FAILED: +1

New test failures (1):
  - pcre-context

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/null-data-dot-newline (2f6926e) with main (d28bf76)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[lhecker]

Thanks!