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

- -L/--files-without-match: print filenames with zero matches (#380)
- --exclude-dir=GLOB: skip directories during recursive grep (#381)
- -s/--no-messages: suppress error messages for nonexistent files (#382)
- -Z/--null: use \0 after filenames instead of : or \n (#383)

Closes #380, closes #381, closes #382, closes #383

Author: claude