### 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 (export_format > EXPORT_FORMAT_AAB || export_format < EXPORT_FORMAT_APK) {
		add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Unsupported export format!"));
		return ERR_UNCONFIGURED;
	}
	if (use_custom_build) {
		print_verbose("Starting custom build...");
		
		{
			print_verbose("Checking build version...");
			Ref<FileAccess> f = FileAccess::open("res://android/.build_version", FileAccess::READ);
			if (f.is_null()) {
				add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Trying to build from a custom built template, but no version info for it exists. Please reinstall from the 'Project' menu."));
				return ERR_UNCONFIGURED;
			}
			String version = f->get_line().strip_edges();
			print_verbose("- build version: " + version);
			if (version != VERSION_FULL_CONFIG) {
				add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), vformat(TTR("Android build version mismatch: Template installed: %s, Godot version: %s. Please reinstall Android build template from 'Project' menu."), version, VERSION_FULL_CONFIG));
				return ERR_UNCONFIGURED;
			}
		}
		const String assets_directory = get_assets_directory(p_preset, export_format);
		String sdk_path = EDITOR_GET("export/android/android_sdk_path");
		ERR_FAIL_COND_V_MSG(sdk_path.is_empty(), ERR_UNCONFIGURED, "Android SDK path must be configured in Editor Settings at 'export/android/android_sdk_path'.");
		print_verbose("Android sdk path: " + sdk_path);
		
		String project_name = get_project_name(p_preset->get("package/name"));
		err = _create_project_name_strings_files(p_preset, project_name); 
		if (err != OK) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Unable to overwrite res://android/build/res/*.xml files with project name."));