### 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' could not be found. Please check that the command is available in the Android SDK build-tools directory. The resulting %s is unsigned."), export_label));
		return OK;
	}
	String keystore;
	String password;
	String user;
	if (p_debug) {
		keystore = p_preset->get("keystore/debug");
		password = p_preset->get("keystore/debug_password");
		user = p_preset->get("keystore/debug_user");
		if (keystore.is_empty()) {
			keystore = EDITOR_GET("export/android/debug_keystore");
			password = EDITOR_GET("export/android/debug_keystore_pass");
			user = EDITOR_GET("export/android/debug_keystore_user");
		}
		if (ep.step(vformat(TTR("Signing debug %s..."), export_label), 104)) {
			return ERR_SKIP;
		}
	} else {
		keystore = release_keystore;
		password = release_password;
		user = release_username;
		if (ep.step(vformat(TTR("Signing release %s..."), export_label), 104)) {
			return ERR_SKIP;
		}
	}
	if (!FileAccess::exists(keystore)) {
		add_message(EXPORT_MESSAGE_WARNING, TTR("Code Signing"), TTR("Could not find keystore, unable to export."));
		return ERR_FILE_CANT_OPEN;
	}