POSIX shell command word parsing and quoting utilities

Shellwords is an OCaml implementation of POSIX shell word parsing, intended to match the behavior of the Rust shell-words crate. The library follows the parsing rules of the POSIX.1-2008 Shell Command Language, including quoting, escaping, line continuations, and comments, while delberately excluding shell expansions such as globbing, variable substitution, command substitution, and arithmetic expansion. The API is designed for argv-style processing and provides functionality such as splitting command strings into words and producing shell-safe representations of arguments. Behavioral compatibility with the Rust shell-words crate is a primary goal.