### 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 (!FileAccess::exists(release_keystore)) {
					add_message(EXPORT_MESSAGE_ERROR, TTR("Code Signing"), TTR("Could not find keystore, unable to export."));
					return ERR_FILE_CANT_OPEN;
				}
				cmdline.push_back("-Prelease_keystore_file=" + release_keystore); 
				cmdline.push_back("-Prelease_keystore_alias=" + release_username); 
				cmdline.push_back("-Prelease_keystore_password=" + release_password); 
			}
		}
		int result = EditorNode::get_singleton()->execute_and_show_output(TTR("Building Android Project (gradle)"), build_command, cmdline);
		if (result != 0) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Building of Android project failed, check output for the error. Alternatively visit docs.godotengine.org for Android build documentation."));
			return ERR_CANT_CREATE;
		}
		List<String> copy_args;
		String copy_command;
		if (export_format == EXPORT_FORMAT_AAB) {
			copy_command = vformat("copyAndRename%sAab", build_type);
		} else if (export_format == EXPORT_FORMAT_APK) {
			copy_command = vformat("copyAndRename%sApk", build_type);
		}
		copy_args.push_back(copy_command);
		copy_args.push_back("-p"); 
		copy_args.push_back(build_path); 
		String export_filename = p_path.get_file();
		String export_path = p_path.get_base_dir();
		if (export_path.is_relative_path()) {
			export_path = OS::get_singleton()->get_resource_dir().path_join(export_path);
		}
		export_path = ProjectSettings::get_singleton()->globalize_path(export_path).simplify_path();