### Godot version

3.5.1.stable, v4.0.beta.custom_build [0a87ead64]

### System information

Ubuntu 22.04.1

### Issue description

For example, if project has name "300spartans", we can't enter the "org.godotengine.300spartans" to "Unique Name" field. We get the error "Invalid package name: A digit cannot be the first character in a package segment". But , if we enter "org.godotengine.$genname", there will be no error message. When exporting, the name will be set as "org.godotengine.spartans" without "300" in beginning, but user will not be informed about it. I think, it is not very good. It may not be worth making changes to the source code, but mention it in the documentation.

### Steps to reproduce

See issue description.

### Minimal reproduction project

N/A
String output;
	List<String> args;
	args.push_back("sign");
	args.push_back("--verbose");
	args.push_back("--ks");
	args.push_back(keystore);
	args.push_back("--ks-pass");
	args.push_back("pass:" + password);
	args.push_back("--ks-key-alias");
	args.push_back(user);
	args.push_back(export_path);
	if (OS::get_singleton()->is_stdout_verbose() && p_debug) {
		
		print_verbose("Signing debug binary using: " + String("\n") + apksigner + " " + join_list(args, String(" ")));
	} else {
		List<String> redacted_args = List<String>(args);
		redacted_args.find(keystore)->set("<REDACTED>");
		redacted_args.find("pass:" + password)->set("pass:<REDACTED>");
		redacted_args.find(user)->set("<REDACTED>");
		print_line("Signing binary using: " + String("\n") + apksigner + " " + join_list(redacted_args, String(" ")));
	}
	int retval;
	Error err = OS::get_singleton()->execute(apksigner, args, &output, &retval, true);
	if (err != OK) {
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), TTR("Could not start apksigner executable."));
		return err;
	}
	
	print_line(output);
	if (retval) {