Match rustc's new shebang handling

[dtolnay]

As of rust-lang/rust#71487 and rust-lang/rust#73596, a file starting with #! is not necessarily a shebang if whitespace/comments after the #! are followed by [, making it an attribute.

#!//am/i/a/comment

fn main() {} // ^ shebang

#!//am/i/a/comment

[allow(dead_code)] // ^ not a shebang
fn main() {}