### 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
bool scale_splash = GLOBAL_GET("application/boot_splash/fullsize");
	bool apply_filter = GLOBAL_GET("application/boot_splash/use_filter");
	String project_splash_path = GLOBAL_GET("application/boot_splash/image");
	if (!project_splash_path.is_empty()) {
		splash_image.instantiate();
		print_verbose("Loading splash image: " + project_splash_path);
		const Error err = ImageLoader::load_image(project_splash_path, splash_image);
		if (err) {
			if (OS::get_singleton()->is_stdout_verbose()) {
				print_error("- unable to load splash image from " + project_splash_path + " (" + itos(err) + ")");
			}
			splash_image.unref();
		}
	}
	if (splash_image.is_null()) {
		
		print_verbose("Using default splash image.");