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

.idea/argument-parser.iml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="CPP_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>