### 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
err += TTR("\"Min SDK\" can only be overridden when \"Use Custom Build\" is enabled.");
			err += "\n";
		}
		if (!min_sdk_str.is_valid_int()) {
			valid = false;
			err += vformat(TTR("\"Min SDK\" should be a valid integer, but got \"%s\" which is invalid."), min_sdk_str);
			err += "\n";
		} else {
			min_sdk_int = min_sdk_str.to_int();
			if (min_sdk_int < DEFAULT_MIN_SDK_VERSION) {
				valid = false;
				err += vformat(TTR("\"Min SDK\" cannot be lower than %d, which is the version needed by the Godot library."), DEFAULT_MIN_SDK_VERSION);
				err += "\n";
			}
		}
	}
	
	String target_sdk_str = p_preset->get("custom_build/target_sdk");
	int target_sdk_int = DEFAULT_TARGET_SDK_VERSION;
	if (!target_sdk_str.is_empty()) { 
		if (!custom_build_enabled) {
			valid = false;
			err += TTR("\"Target SDK\" can only be overridden when \"Use Custom Build\" is enabled.");
			err += "\n";
		}
		if (!target_sdk_str.is_valid_int()) {
			valid = false;
			err += vformat(TTR("\"Target SDK\" should be a valid integer, but got \"%s\" which is invalid."), target_sdk_str);
			err += "\n";
		} else {