### 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
String err;
	bool valid = true;
	const bool custom_build_enabled = p_preset->get("custom_build/use_custom_build");
	
	bool apk_expansion = p_preset->get("apk_expansion/enable");
	if (apk_expansion) {
		String apk_expansion_pkey = p_preset->get("apk_expansion/public_key");
		if (apk_expansion_pkey.is_empty()) {
			valid = false;
			err += TTR("Invalid public key for APK expansion.") + "\n";
		}
	}
	String pn = p_preset->get("package/unique_name");
	String pn_err;
	if (!is_package_name_valid(get_package_name(pn), &pn_err)) {
		valid = false;
		err += TTR("Invalid package name:") + " " + pn_err + "\n";
	}
	String etc_error = test_etc2();
	if (!etc_error.is_empty()) {
		valid = false;
		err += etc_error;
	}
	
	String enabled_plugins_names = PluginConfigAndroid::get_plugins_names(get_enabled_plugins(p_preset));
	if (!enabled_plugins_names.is_empty() && !custom_build_enabled) {
		valid = false;
		err += TTR("\"Use Custom Build\" must be enabled to use the plugins.");