### 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
unz_file_info info;
		char fname[16384];
		ret = unzGetCurrentFileInfo(pkg, &info, fname, 16384, nullptr, 0, nullptr, 0);
		if (ret != UNZ_OK) {
			break;
		}
		bool skip = false;
		String file = String::utf8(fname);
		Vector<uint8_t> data;
		data.resize(info.uncompressed_size);
		
		unzOpenCurrentFile(pkg);
		unzReadCurrentFile(pkg, data.ptrw(), data.size());
		unzCloseCurrentFile(pkg);
		
		if (file == "AndroidManifest.xml") {
			_fix_manifest(p_preset, data, p_give_internet);
		}
		if (file == "resources.arsc") {
			_fix_resources(p_preset, data);
		}
		
		if ((file == SPLASH_IMAGE_EXPORT_PATH || file == LEGACY_BUILD_SPLASH_IMAGE_EXPORT_PATH) && splash_image.is_valid() && !splash_image->is_empty()) {
			_load_image_data(splash_image, data);
		}
		
		if ((file == SPLASH_BG_COLOR_PATH || file == LEGACY_BUILD_SPLASH_BG_COLOR_PATH) && splash_bg_color_image.is_valid() && !splash_bg_color_image->is_empty()) {
			_load_image_data(splash_bg_color_image, data);
		}