modernization: tweaks for c++20

Update various places for what we can do with C++20:

- ends_with / starts_with
- std::to_array
- using instead of typedef
- designated initializers
- "[[maybe_unused]]" instead of G_GNUC_UNUSED

Also remove some unnecessary 'static'

mu/mu-options.cc

@@ -716,7 +716,7 @@ struct CommandInfo {
 	std::string_view	help;
 
 	// std::function is not constexp-friendly
-	typedef void(*setup_func_t)(CLI::App&, Options&);
+	using setup_func_t = void(*)(CLI::App&, Options&);
 	setup_func_t setup_func{};
 };