Proposal static type constraints features

[YU HengChun]

This is easy to solve, we change the word. But the code will be longer.

The original:

```js
function f(x) {
  return typeof x !== 'string' || typeof x === 'undefined'; 
}
```

The proposal: typeof version

```js
function f(x = typeof(String) && undefined ) { // Attention can not be omitted: && undefined
  void typeof(Boolean); // or only: typeof(Boolean);
  return typeof x !== 'string' || typeof x === 'undefined'; 
}
```

这很容易解决, 换个单词就行. 但是代码会更长.

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