### 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
int passthrough_mode = p_preset->get("xr_features/passthrough");
						if (passthrough_mode > XR_PASSTHROUGH_NONE) {
							feature_names.push_back("com.oculus.feature.PASSTHROUGH");
							feature_required_list.push_back(passthrough_mode == XR_PASSTHROUGH_REQUIRED);
							feature_versions.push_back(-1);
						}
					}
					if (feature_names.size() > 0) {
						ofs += 24; 
						
						Vector<uint8_t> manifest_end;
						uint32_t manifest_cur_size = p_manifest.size();
						manifest_end.resize(p_manifest.size() - ofs);
						memcpy(manifest_end.ptrw(), &p_manifest[ofs], manifest_end.size());
						int32_t attr_name_string = string_table.find("name");
						ERR_FAIL_COND_MSG(attr_name_string == -1, "Template does not have 'name' attribute.");
						int32_t ns_android_string = string_table.find("http://schemas.android.com/apk/res/android");
						if (ns_android_string == -1) {
							string_table.push_back("http://schemas.android.com/apk/res/android");
							ns_android_string = string_table.size() - 1;
						}
						int32_t attr_uses_feature_string = string_table.find("uses-feature");
						if (attr_uses_feature_string == -1) {
							string_table.push_back("uses-feature");
							attr_uses_feature_string = string_table.size() - 1;
						}
						int32_t attr_required_string = string_table.find("required");
						if (attr_required_string == -1) {
							string_table.push_back("required");
							attr_required_string = string_table.size() - 1;