### 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
if (tname == "meta-data" && attrname == "name" && value == "xr_hand_tracking_version_name") {
							string_table.write[attr_value] = "com.oculus.handtracking.version";
						}
						if (tname == "meta-data" && attrname == "value" && value == "xr_hand_tracking_version_value") {
							string_table.write[attr_value] = "V2.0";
						}
					}
					iofs += 20;
				}
			} break;
			case CHUNK_XML_END_TAG: {
				int iofs = ofs + 8;
				uint32_t name = decode_uint32(&p_manifest[iofs + 12]);
				String tname = string_table[name];
				if (tname == "uses-feature") {
					Vector<String> feature_names;
					Vector<bool> feature_required_list;
					Vector<int> feature_versions;
					if (xr_mode_index == XR_MODE_OPENXR) {
						
						feature_names.push_back("android.hardware.vr.headtracking");
						feature_required_list.push_back(true);
						feature_versions.push_back(1);
						
						if (hand_tracking_index > XR_HAND_TRACKING_NONE) {
							feature_names.push_back("oculus.software.handtracking");
							feature_required_list.push_back(hand_tracking_index == XR_HAND_TRACKING_REQUIRED);
							feature_versions.push_back(-1); 
						}