### 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
versions.push_back(sub_dir);
			} else {
				if (ver_number >= min_version && ver_number <= DEFAULT_TARGET_SDK_VERSION) {
					ideal_versions.push_back(sub_dir);
				} else {
					other_versions.push_back(sub_dir);
				}
			}
		}
	}
	
	versions.append_array(ideal_versions);
	versions.append_array(other_versions);
	if (!versions.size()) {
		print_error("Unable to find the 'apksigner' tool.");
		return apksigner_path;
	}
	int i;
	bool failed = false;
	String version_to_use;
	List<String> args;
	args.push_back("--version");
	String output;
	int retval;
	Error err;
	for (i = 0; i < versions.size(); i++) {
		
		apksigner_path = build_tools_dir.path_join(versions[i]).path_join(apksigner_command_name);
		if (FileAccess::exists(apksigner_path)) {
			version_to_use = versions[i];