Add GDScript slave keyword deprecation warning.

modules/gdscript/gdscript_parser.cpp

@@ -4443,6 +4443,7 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 				continue;
 			} break;
 			case GDScriptTokenizer::TK_PR_SLAVE:
+				_add_warning(GDScriptWarning::DEPRECATED_KEYWORD, tokenizer->get_token_line(), "slave", "puppet");
 			case GDScriptTokenizer::TK_PR_PUPPET: {
 
 				//may be fallthrough from export, ignore if so