Handle multiple spaces between fn and main

[YetForge]

Addresses one case from #150

rust-script currently allows multiple spaces between main and (, but its
main-detection regex still requires exactly one space between fn and main.

As a result, valid Rust such as:

fn   main() {
    println!("Hello, World!");
}

is not detected as an existing main function and is wrapped incorrectly.

This change allows one or more spaces between fn and main while preserving
the existing detection behavior for the other supported main forms.

A regression test is added for the multi-space form.