feat(builtins): add grep -L, --exclude-dir, -s, -Z flags (#385)

## Summary

- Add `-L`/`--files-without-match` flag: print filenames that have zero
matches (inverse of `-l`) - Closes #380
- Add `--exclude-dir=GLOB` flag: skip entire directories during
recursive grep - Closes #381
- Add `-s`/`--no-messages` flag: suppress error messages about
nonexistent/unreadable files - Closes #382
- Add `-Z`/`--null` flag: print `\0` after filenames instead of `:` or
`\n` (for `-l`/`-L` output and filename prefixes) - Closes #383

## Test plan

- [x] Unit tests for each flag (positive and negative cases)
- [x] Spec tests for each flag in `grep.test.sh`
- [x] `cargo fmt --check` passes
- [x] `cargo clippy --all-targets --all-features -- -D warnings` passes
- [x] `cargo test --all-features -p bashkit --lib` passes (1126 tests)
- [x] `cargo test --all-features -p bashkit --test spec_tests -- grep`
passes (82 spec tests)

Co-authored-by: Claude <noreply@anthropic.com>

Author: chaliy