Do not lint write_with_newline when there is more than one newline

My code uses:

```rust
write!(file, "// THIS FILE IS AUTOMATICALLY GENERATED; DO NOT EDIT\n\n")?;
```

I believe the above is clearer than Clippy's suggestion, in which one newline is written by the macro and one newline is written by my string:

```rust
writeln!(file, "// THIS FILE IS AUTOMATICALLY GENERATED; DO NOT EDIT\n")?;
```

Mentioning @estk who worked on this lint in #2630.