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 |
```

Start by reading the throttle implementation and its existing options, then inspect the current tests for leading and trailing calls. Add coverage for the trailing:false behavior shown in the issue and verify that trailing calls are suppressed while the leading calls still occur as illustrated.

Written by the indexing model from the issue text.