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-cmd-view.cc

@@ -121,7 +121,7 @@ body_or_summary(const Message& message, const Options& opts)
 		print_field("Summary", summ, color);
 	} else {
 		mu_print_encoded("{}", body);
-		if (!g_str_has_suffix(body.c_str(), "\n"))
+		if (!body.ends_with('\n'))
 			mu_println("");
 	}
 }

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{};
 };
 

mu/tests/test-mu-query.cc

@@ -111,10 +111,10 @@ run_and_count_matches(const std::string& xpath,
 	return qres->size();
 }
 
-typedef struct {
+struct QResults {
 	const char* query;
 	size_t      count; /* expected number of matches */
-} QResults;
+};
 
 static void
 test_mu_query_01(void)