### 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
int export_format = int(p_preset->get("custom_build/export_format"));
	String export_label = export_format == EXPORT_FORMAT_AAB ? "AAB" : "APK";
	String release_keystore = p_preset->get("keystore/release");
	String release_username = p_preset->get("keystore/release_user");
	String release_password = p_preset->get("keystore/release_password");
	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 apksigner = get_apksigner_path(target_sdk_version.to_int(), true);
	print_verbose("Starting signing of the " + export_label + " binary using " + apksigner);
	if (apksigner == "<FAILED>") {
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), vformat(TTR("All 'apksigner' tools located in Android SDK 'build-tools' directory failed to execute. Please check that you have the correct version installed for your target sdk version. The resulting %s is unsigned."), export_label));
		return OK;
	}
	if (!FileAccess::exists(apksigner)) {