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("");
 	}
 }