Single-line multiline strings not containing " should be disallowed

### Description

Multiline strings that are a single line and do not contain the `"` character should be disallowed. This seems incorrect, we should insist on single-line strings where they apply.

### Small example input

```nix
''Hello world!''
```

### Expected output

```nix
"Hello world!"
```

### Actual output

```nix
''Hello world!''
```
