Refactor ARGV handling and add `system.programArgs`

C API Changes:

• A new gjs_context_set_argv function is introduced for API user

JS Changes:

• imports.system is renamed to imports._systemNative
• imports.system is now a JS script
• imports._systemNative.buildProgramArgs() builds the ARGV array
• system.programArgs is set with buildProgramArgs and ARGV is a getter for system.programArgs.

To avoid breaking backwards compatibility with API users, gjs_context_define_string_array redirects "ARGV" to the new implementation.

Goals:

• Use std::string and std::vector for ARGV handling where ever possible (we can't use them in the public headers)
• Avoid using gjs_context_define_string_array for ARGV. This method assumes that the global will be defined before a realm is entered. This will have to continue for the current global, but we don't need to further the dependency.
• Ensure ARGV is defined in all GJS contexts, regardless of if an API user calls gjs_context_define_string_array and sets it up.

Benefits:

• Uses modern C++ instead of C-style string/memory handling
• Avoids pre-realm global dependencies
• Global scope consistency across implementations
• API users don't have to setup ARGV themselves