Add support for `trailing:false`

The current default behavior is for two quick calls to result in two slow calls: one leading call and one trailing call. This is often not desirable - the point of throttling is to _reduce_ the number of calls, not just to slow them down. For this we would need a `trailing` option to control trailing edge calls:

```
| fn() | 1 2 3 4 |
| throttle(fn, 100) | 1 3 4 |
| throttle(fn, 100, {trailing: false}) | 1 3 |
```

首先阅读 throttle 的实现及其现有选项，然后检查当前针对 leading 和 trailing 调用的测试。为 issue 中所示的 trailing:false 行为添加覆盖，并验证 trailing 调用会被抑制，而 leading 调用仍会如图所示发生。

由索引模型根据 Issue 内容生成。