### 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
String build_tools_dir = sdk_path.path_join("build-tools");
	Ref<DirAccess> da = DirAccess::open(build_tools_dir, &errn);
	if (errn != OK) {
		print_error("Unable to open Android 'build-tools' directory.");
		return apksigner_path;
	}
	
	Vector<String> dir_list = da->get_directories();
	
	
	
	
	Vector<String> ideal_versions;
	Vector<String> other_versions;
	Vector<String> versions;
	bool found_target_sdk = false;
	
	int min_version = p_target_sdk <= 27 ? p_target_sdk : 28;
	for (String sub_dir : dir_list) {
		if (!sub_dir.begins_with(".")) {
			Vector<String> ver_numbers = sub_dir.split(".");
			
			if (!ver_numbers.size() || !ver_numbers[0].is_valid_int()) {
				other_versions.push_back(sub_dir);
				continue;
			}
			int ver_number = ver_numbers[0].to_int();
			if (ver_number == p_target_sdk) {
				found_target_sdk = true;