### 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
add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), vformat(TTR("'apksigner' returned with error #%d"), retval));
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), vformat(TTR("output: \n%s"), output));
		return ERR_CANT_CREATE;
	}
	if (ep.step(vformat(TTR("Verifying %s..."), export_label), 105)) {
		return ERR_SKIP;
	}
	args.clear();
	args.push_back("verify");
	args.push_back("--verbose");
	args.push_back(export_path);
	if (p_debug) {
		print_verbose("Verifying signed build using: " + String("\n") + apksigner + " " + join_list(args, String(" ")));
	}
	output.clear();
	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_verbose(output);
	if (retval) {
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), vformat(TTR("'apksigner' verification of %s failed."), export_label));
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), vformat(TTR("output: \n%s"), output));
		return ERR_CANT_CREATE;
	}
	print_verbose("Successfully completed signing build.");
	return OK;
}
void EditorExportPlatformAndroid::_clear_assets_directory() {