### 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 += "\n";
	}
	
	int xr_mode_index = p_preset->get("xr_features/xr_mode");
	int hand_tracking = p_preset->get("xr_features/hand_tracking");
	int passthrough_mode = p_preset->get("xr_features/passthrough");
	if (xr_mode_index != XR_MODE_OPENXR) {
		if (hand_tracking > XR_HAND_TRACKING_NONE) {
			valid = false;
			err += TTR("\"Hand Tracking\" is only valid when \"XR Mode\" is \"OpenXR\".");
			err += "\n";
		}
		if (passthrough_mode > XR_PASSTHROUGH_NONE) {
			valid = false;
			err += TTR("\"Passthrough\" is only valid when \"XR Mode\" is \"OpenXR\".");
			err += "\n";
		}
	}
	if (int(p_preset->get("custom_build/export_format")) == EXPORT_FORMAT_AAB &&
			!custom_build_enabled) {
		valid = false;
		err += TTR("\"Export AAB\" is only valid when \"Use Custom Build\" is enabled.");
		err += "\n";
	}
	
	String min_sdk_str = p_preset->get("custom_build/min_sdk");
	int min_sdk_int = DEFAULT_MIN_SDK_VERSION;
	if (!min_sdk_str.is_empty()) { 
		if (!custom_build_enabled) {
			valid = false;