Register std::string_view as the "string_view" user type with a built-in
implicit conversion from std::string, so a ChaiScript string can be passed
directly to a C++ function taking std::string_view. Adds the reverse
explicit conversion via string(sv) / to_string(sv), plus basic queries
(size, length, empty, data) and comparison operators on string_view.
String-style methods that take size_t (substr, find, ...) are intentionally
not duplicated on string_view: with the implicit conversion in place they
would create dispatch ambiguity for calls like string.substr(int, int).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
