### 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
print_verbose("- custom build enabled: " + bool_to_string(use_custom_build));
	print_verbose("- apk expansion enabled: " + bool_to_string(apk_expansion));
	print_verbose("- enabled abis: " + join_abis(enabled_abis, ",", false));
	print_verbose("- export filter: " + itos(p_preset->get_export_filter()));
	print_verbose("- include filter: " + p_preset->get_include_filter());
	print_verbose("- exclude filter: " + p_preset->get_exclude_filter());
	Ref<Image> splash_image;
	Ref<Image> splash_bg_color_image;
	String processed_splash_config_xml = load_splash_refs(splash_image, splash_bg_color_image);
	Ref<Image> main_image;
	Ref<Image> foreground;
	Ref<Image> background;
	load_icon_refs(p_preset, main_image, foreground, background);
	Vector<uint8_t> command_line_flags;
	
	get_command_line_flags(p_preset, p_path, p_flags, command_line_flags);
	if (export_format == EXPORT_FORMAT_AAB) {
		if (!p_path.ends_with(".aab")) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Invalid filename! Android App Bundle requires the *.aab extension."));
			return ERR_UNCONFIGURED;
		}
		if (apk_expansion) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("APK Expansion not compatible with Android App Bundle."));
			return ERR_UNCONFIGURED;
		}
	}
	if (export_format == EXPORT_FORMAT_APK && !p_path.ends_with(".apk")) {
		add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Invalid filename! Android APK requires the *.apk extension."));
		return ERR_UNCONFIGURED;
	}