Allow marking @record types as @final to disallow undeclared fields

Use case: "keyword" arguments to functions. It would be wonderful to get an error for a typo-ed optional argument key.

``` js
/** @record @final */ const ExtraArgs = function() {};
/** @type {!string|undefined} */ ExtraArgs.prototype.foo;
```

Then `{}` and `{foo: 'bar'}` would both match, but `{fop: 'bar'}` would fail to match because the interface is sealed and `fop` isn't a known field.