Support for shebang

[blt-r]

rustc already can ignore the shebang. And cargo-script RFC was accepted (rust-lang/cargo#12207).

This is a valid rust:

#!/usr/bin/env -S cargo +nightly -Zscript

fn main() {
    println!("Hello from cargo script")
}

But it isn't parsed

[blt-r]

It is also documented in https://doc.rust-lang.org/reference/crates-and-source-files.html#shebang