refactor(parser): improve code style and modernize macos parser

- Remove unnecessary whitespace in argparse.hpp
- Modernize macos_parser.hpp with if-initializer
- Clean up includes and add [[nodiscard]] attributes
- Add .idea files to gitignore

include/argparse

@@ -2,7 +2,7 @@
 #ifndef ARGPARSE_HPP
 #define ARGPARSE_HPP
 #include <argument_parser.hpp>
-#
+
 #ifdef __linux__
 #include <linux_parser.hpp>
 namespace argument_parser {