grep: support \s and \S in BRE mode

[wondr-wclabs]

Closes #31.

This enables Oniguruma's whitespace escape operator for RegexMode::Basic so GNU BRE extensions \s and \S are recognized in the default -G mode. The change is intentionally scoped to BRE: fixed-string mode should stay literal, ERE already uses Syntax::gnu_regex(), and PCRE has its own syntax path.

I chose the Onig syntax flag rather than pre-processing patterns because it keeps escaping semantics inside the regex engine and avoids hand-maintaining a second parser for these shorthands. The new test cases live in the existing bre_gnu_extensions coverage alongside other GNU BRE extensions.

Validation run locally:

• cargo fmt --all -- --check
• cargo test bre_gnu_extensions -- --nocapture
• printf 'a b\nxy\n' | cargo run --quiet -- -e '\s' -> a b
• printf 'aS b\n \nx\n' | cargo run --quiet -- -c '\S' -> 2
• cargo test
• cargo test --no-fail-fast
• cargo clippy --all-targets --workspace -p uu_grep -- -D warnings
• git diff --check

[wondr-wclabs]

Closing this as a duplicate of #37. While re-checking the issue list after publishing, I saw that #37 already targets the same GNU BRE / compatibility gap for #31. Leaving two PRs against the same issue would add review noise without a distinct behavior change, so this one should not stay open.