Reintroduce `Expect.true/false` with a better API?

Maybe we should make it easier to do this:
```elm
|> Expect.equal True
|> Expect.onFail "The URL should still be present in the final URL after running autolinksUrl"
```
by having something like
```elm
|> Expect.true { onFalse = "The URL ..." }
```

The previous maintainers' reasons for removing `Expect.true` was that it took away the spotlight from better solutions (`Expect.equalLists`, ...), but sometimes you just have to work with predicates. It's not always possible/practical to use the other helpers. Let's trust our users to understand when to use which solution?