### 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
print_verbose("Storing command line flags...");
		store_file_at_path(assets_directory + "/_cl_", command_line_flags);
		print_verbose("Updating ANDROID_HOME environment to " + sdk_path);
		OS::get_singleton()->set_environment("ANDROID_HOME", sdk_path); 
		String build_command;
#ifdef WINDOWS_ENABLED
		build_command = "gradlew.bat";
#else
		build_command = "gradlew";
#endif
		String build_path = ProjectSettings::get_singleton()->get_resource_path().path_join("android/build");
		build_command = build_path.path_join(build_command);
		String package_name = get_package_name(p_preset->get("package/unique_name"));
		String version_code = itos(p_preset->get("version/code"));
		String version_name = p_preset->get("version/name");
		String min_sdk_version = p_preset->get("custom_build/min_sdk");
		if (!min_sdk_version.is_valid_int()) {
			min_sdk_version = itos(DEFAULT_MIN_SDK_VERSION);
		}
		String target_sdk_version = p_preset->get("custom_build/target_sdk");
		if (!target_sdk_version.is_valid_int()) {
			target_sdk_version = itos(DEFAULT_TARGET_SDK_VERSION);
		}
		String enabled_abi_string = join_abis(enabled_abis, "|", false);
		String sign_flag = should_sign ? "true" : "false";
		String zipalign_flag = "true";
		Vector<PluginConfigAndroid> enabled_plugins = get_enabled_plugins(p_preset);
		String local_plugins_binaries = PluginConfigAndroid::get_plugins_binaries(PluginConfigAndroid::BINARY_TYPE_LOCAL, enabled_plugins);
		String remote_plugins_binaries = PluginConfigAndroid::get_plugins_binaries(PluginConfigAndroid::BINARY_TYPE_REMOTE, enabled_plugins);
		String custom_maven_repos = PluginConfigAndroid::get_plugins_custom_maven_repos(enabled_plugins);