grep: honor whitespace escapes in BRE

[wondr-wclabs]

Fixes #31.

This enables Oniguruma's \s/\S whitespace shorthand operator for basic-regexp mode. The rest of the BRE syntax stays unchanged; this only fills the documented gap where \w/\W already worked in BRE but \s/\S were treated as literals.

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 71 / FAILED: 36 / 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):
  - backslash-s-vs-invalid-multibyte

Test improvements (2):
  + backslash-s-and-repetition-operators
  + multibyte-white-space

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/bre-whitespace-escapes (b2dd516) 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. ↩

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 74 / FAILED: 33 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 72 / FAILED: 35 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +2
  FAILED: -2

Test improvements (2):
  + backslash-s-and-repetition-operators
  + multibyte-white-space

[wondr-wclabs]

Closing this older duplicate in favor of #55. The replacement PR keeps the same GNU \s/\S BRE compatibility goal, but its final implementation accounts for the GNU invalid-multibyte test: it expands BRE whitespace shorthands to POSIX space classes instead of enabling Oniguruma's shorthand operator directly, so \S does not match invalid UTF-8 bytes.