lang: implement elif keyword for more complex if-statements

lang/ast/block.c

@@ -160,6 +160,7 @@ struct ast_node_type block_node_ops = {
 		KW_PARSER(VAR, parse_var),
 		KW_PARSER(END, parse_end),
 		KW_PARSER(ELSE, parse_else),
+		KW_PARSER(ELIF, parse_else), /* same behaviour as ELSE */
 		KW_PARSER_FALLBACK(parse_keyword),
 	},
 	.n_symbol_parsers = {