### 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
encode_uint32(retain_data_on_uninstall, &p_manifest.write[iofs + 16]);
					}
					if (tname == "activity" && attrname == "screenOrientation") {
						encode_uint32(screen_orientation, &p_manifest.write[iofs + 16]);
					}
					if (tname == "activity" && attrname == "excludeFromRecents") {
						encode_uint32(exclude_from_recents, &p_manifest.write[iofs + 16]);
					}
					if (tname == "activity" && attrname == "resizeableActivity") {
						encode_uint32(is_resizeable, &p_manifest.write[iofs + 16]);
					}
					if (tname == "supports-screens") {
						if (attrname == "smallScreens") {
							encode_uint32(screen_support_small ? 0xFFFFFFFF : 0, &p_manifest.write[iofs + 16]);
						} else if (attrname == "normalScreens") {
							encode_uint32(screen_support_normal ? 0xFFFFFFFF : 0, &p_manifest.write[iofs + 16]);
						} else if (attrname == "largeScreens") {
							encode_uint32(screen_support_large ? 0xFFFFFFFF : 0, &p_manifest.write[iofs + 16]);
						} else if (attrname == "xlargeScreens") {
							encode_uint32(screen_support_xlarge ? 0xFFFFFFFF : 0, &p_manifest.write[iofs + 16]);
						}
					}
					
					if (xr_mode_index == XR_MODE_OPENXR && hand_tracking_index > XR_HAND_TRACKING_NONE) {
						if (tname == "meta-data" && attrname == "name" && value == "xr_hand_tracking_metadata_name") {
							string_table.write[attr_value] = "com.oculus.handtracking.frequency";
						}
						if (tname == "meta-data" && attrname == "value" && value == "xr_hand_tracking_metadata_value") {
							string_table.write[attr_value] = (hand_tracking_frequency_index == XR_HAND_TRACKING_FREQUENCY_LOW ? "LOW" : "HIGH");
						}