feat: grep -R/-r flag

This is a feature request to add the -r or -R flag to grep(). From the man page:

-r, --recursive
       Read  all  files under each directory, recursively, following symbolic links only if they are on the command line.  Note that if no file operand is given, grep searches the working directory.  This is equivalent
       to the -d recurse option.

-R, --dereference-recursive
       Read all files under each directory, recursively.  Follow all symbolic links, unlike -r.

The issue names the grep() entry point but no implementation file or test. Start by locating grep() and its option parsing, then inspect existing grep behavior and tests. Done means -r and -R are accepted and follow the recursive and symbolic-link semantics described in the request.