Replace the current signal API with one following a similar model to
EventTarget, a la Deno.

Before

```
const sh = tjs.signal('SIGINT', handleSigInt);
// sh needs to be alive or the signal handler is destroyed.
```

After

```
tjs.addSignalListener('SIGINT', handleSigInt);
// No need to use a handle, it can be removed with
// tjs.removeSignalListener.
```

Fixes: #476
Ref: #477

Ref: #477