### 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
if (!DirAccess::exists(p_path.get_base_dir())) {
		return ERR_FILE_BAD_PATH;
	}
	Ref<FileAccess> io_fa;
	zlib_filefunc_def io = zipio_create_io(&io_fa);
	if (ep.step(TTR("Creating APK..."), 0)) {
		return ERR_SKIP;
	}
	unzFile pkg = unzOpen2(src_apk.utf8().get_data(), &io);
	if (!pkg) {
		add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), vformat(TTR("Could not find template APK to export: \"%s\"."), src_apk));
		return ERR_FILE_NOT_FOUND;
	}
	int ret = unzGoToFirstFile(pkg);
	Ref<FileAccess> io2_fa;
	zlib_filefunc_def io2 = zipio_create_io(&io2_fa);
	String tmp_unaligned_path = EditorPaths::get_singleton()->get_cache_dir().path_join("tmpexport-unaligned." + uitos(OS::get_singleton()->get_unix_time()) + ".apk");
#define CLEANUP_AND_RETURN(m_err)                            \
	{                                                        \
		DirAccess::remove_file_or_error(tmp_unaligned_path); \
		return m_err;                                        \
	}                                                        \
	((void)0)
	zipFile unaligned_apk = zipOpen2(tmp_unaligned_path.utf8().get_data(), APPEND_STATUS_CREATE, nullptr, &io2);
	String cmdline = p_preset->get("command_line/extra_args");
	String version_name = p_preset->get("version/name");
	String package_name = p_preset->get("package/unique_name");
	String apk_expansion_pkey = p_preset->get("apk_expansion/public_key");
	Vector<ABI> invalid_abis(enabled_abis);
	while (ret == UNZ_OK) {