Add mod declaration after the last availaible in the file when performing "Attach file to main.rs" action

[avrong]

Environment

• IntelliJ Rust plugin version: 0.3.127.3249-202
• Rust toolchain version: 1.45.2 (d3fb005a3 2020-07-31) x86_64-apple-darwin
• IDE name and version: CLion 2020.2 (CL-202.6397.106)
• Operating system: macOS 10.15.6

Problem description

I believe that the better option will be to add the new mod declaration right after the last one available if it is present. It will reduce the following manual editing.

Optimize Imports action doesn't solve the problem completely.

Expected result

use std::path::Path;

use module::whatever;

mod module;
mod new_module;

fn main() {...}

[Kobzol]

I wanted to do this in the original PR, but didn't get to it. I think that this is a good idea.