Improve class parser (#289)

- accept `class P { async = 1 }}`
- accept `class P { static = 1 }}` etc.
- Fixes #261

tests/test_language.js

@@ -335,11 +335,13 @@ function test_class()
     assert(S.x === 42);
     assert(S.y === 42);
     assert(S.z === 42);
-    
+
     class P {
-      get = () => "123"
+        get = () => "123";
+        static() { return 42; }
     }
     assert(new P().get() === "123");
+    assert(new P().static() === 42);
 };
 
 function test_template()