Does not detect sentences ending with ! or ?

Right now only `.` is detected:

https://github.com/JoshuaKGoldberg/sentences-per-line/blob/bea2b68340989f637bda693fea548bd874a029be/index.js#L88

So this test case does not work:

```md
Hello world! Another sentence!
```

How unfortunate.