### 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
r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "custom_template/debug", PROPERTY_HINT_GLOBAL_FILE, "*.apk"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "custom_template/release", PROPERTY_HINT_GLOBAL_FILE, "*.apk"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::BOOL, "custom_build/use_custom_build"), false));
	r_options->push_back(ExportOption(PropertyInfo(Variant::INT, "custom_build/export_format", PROPERTY_HINT_ENUM, "Export APK,Export AAB"), EXPORT_FORMAT_APK));
	
	
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "custom_build/min_sdk", PROPERTY_HINT_PLACEHOLDER_TEXT, vformat("%d (default)", DEFAULT_MIN_SDK_VERSION)), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "custom_build/target_sdk", PROPERTY_HINT_PLACEHOLDER_TEXT, vformat("%d (default)", DEFAULT_TARGET_SDK_VERSION)), ""));
	Vector<PluginConfigAndroid> plugins_configs = get_plugins();
	for (int i = 0; i < plugins_configs.size(); i++) {
		print_verbose("Found Android plugin " + plugins_configs[i].name);
		r_options->push_back(ExportOption(PropertyInfo(Variant::BOOL, vformat("%s/%s", PNAME("plugins"), plugins_configs[i].name)), false));
	}
	plugins_changed.clear();
	
	
	const Vector<ABI> abis = get_abis();
	for (int i = 0; i < abis.size(); ++i) {
		const String abi = abis[i].abi;
		
		
		const bool is_default = abi == "arm64-v8a";
		r_options->push_back(ExportOption(PropertyInfo(Variant::BOOL, vformat("%s/%s", PNAME("architectures"), abi)), is_default));
	}
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "keystore/debug", PROPERTY_HINT_GLOBAL_FILE, "*.keystore,*.jks"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "keystore/debug_user"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "keystore/debug_password"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "keystore/release", PROPERTY_HINT_GLOBAL_FILE, "*.keystore,*.jks"), ""));
	r_options->push_back(ExportOption(PropertyInfo(Variant::STRING, "keystore/release_user"), ""));