### 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
bool clean_build_required = is_clean_build_required(enabled_plugins);
		List<String> cmdline;
		if (clean_build_required) {
			cmdline.push_back("clean");
		}
		String build_type = p_debug ? "Debug" : "Release";
		if (export_format == EXPORT_FORMAT_AAB) {
			String bundle_build_command = vformat("bundle%s", build_type);
			cmdline.push_back(bundle_build_command);
		} else if (export_format == EXPORT_FORMAT_APK) {
			String apk_build_command = vformat("assemble%s", build_type);
			cmdline.push_back(apk_build_command);
		}
		cmdline.push_back("-p"); 
		cmdline.push_back(build_path); 
		cmdline.push_back("-Pexport_package_name=" + package_name); 
		cmdline.push_back("-Pexport_version_code=" + version_code); 
		cmdline.push_back("-Pexport_version_name=" + version_name); 
		cmdline.push_back("-Pexport_version_min_sdk=" + min_sdk_version); 
		cmdline.push_back("-Pexport_version_target_sdk=" + target_sdk_version); 
		cmdline.push_back("-Pexport_enabled_abis=" + enabled_abi_string); 
		cmdline.push_back("-Pplugins_local_binaries=" + local_plugins_binaries); 
		cmdline.push_back("-Pplugins_remote_binaries=" + remote_plugins_binaries); 
		cmdline.push_back("-Pplugins_maven_repos=" + custom_maven_repos); 
		cmdline.push_back("-Pperform_zipalign=" + zipalign_flag); 
		cmdline.push_back("-Pperform_signing=" + sign_flag); 
		cmdline.push_back("-Pgodot_editor_version=" + String(VERSION_FULL_CONFIG));