### 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
const String value = (attr_value != 0xFFFFFFFF) ? string_table[attr_value] : "Res #" + itos(attr_resid);
					String attrname = string_table[attr_name];
					const String nspace = (attr_nspace != 0xFFFFFFFF) ? string_table[attr_nspace] : "";
					
					if (tname == "manifest" && attrname == "package") {
						string_table.write[attr_value] = get_package_name(package_name);
					}
					if (tname == "manifest" && attrname == "versionCode") {
						encode_uint32(version_code, &p_manifest.write[iofs + 16]);
					}
					if (tname == "manifest" && attrname == "versionName") {
						if (attr_value == 0xFFFFFFFF) {
							WARN_PRINT("Version name in a resource, should be plain text");
						} else {
							string_table.write[attr_value] = version_name;
						}
					}
					if (tname == "application" && attrname == "requestLegacyExternalStorage") {
						encode_uint32(has_read_write_storage_permission ? 0xFFFFFFFF : 0, &p_manifest.write[iofs + 16]);
					}
					if (tname == "application" && attrname == "allowBackup") {
						encode_uint32(backup_allowed, &p_manifest.write[iofs + 16]);
					}
					if (tname == "application" && attrname == "appCategory") {
						encode_uint32(_get_app_category_value(app_category), &p_manifest.write[iofs + 16]);
					}
					if (tname == "application" && attrname == "isGame") {
						encode_uint32(app_category == APP_CATEGORY_GAME, &p_manifest.write[iofs + 16]);
					}
					if (tname == "application" && attrname == "hasFragileUserData") {