Raise error if appending after #end_with called

Raises an error if a user attempts to append more to the regex after #end_with has already been called:

```
Regularity.new
  .start_with(3, :digits)
  .end_with('$')
  .then('-')  # raises Regularity::Error
```

This provides symmetry to the error raised if #start_with gets called and it's not the beginning of the regex.