### 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
}
						for (int i = 0; i < feature_names.size(); i++) {
							String feature_name = feature_names[i];
							bool feature_required = feature_required_list[i];
							int feature_version = feature_versions[i];
							bool has_version_attribute = feature_version != -1;
							print_line("Adding feature " + feature_name);
							int32_t feature_string = string_table.find(feature_name);
							if (feature_string == -1) {
								string_table.push_back(feature_name);
								feature_string = string_table.size() - 1;
							}
							String required_value_string = feature_required ? "true" : "false";
							int32_t required_value = string_table.find(required_value_string);
							if (required_value == -1) {
								string_table.push_back(required_value_string);
								required_value = string_table.size() - 1;
							}
							int32_t attr_version_string = -1;
							int32_t version_value = -1;
							int tag_size;
							int attr_count;
							if (has_version_attribute) {
								attr_version_string = string_table.find("version");
								if (attr_version_string == -1) {
									string_table.push_back("version");
									attr_version_string = string_table.size() - 1;
								}
								version_value = string_table.find(itos(feature_version));
								if (version_value == -1) {