Add missing AssocItems in add_custom_impl assist

[Veykril]

use std::fmt;

#[derive(Debu<|>g)]
struct Foo {
    bar: String,
}

->

use std::fmt;

struct Foo {
    bar: String,
}

impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ${0:todo!()}
    }
}

[matklad]

Nice, I've wanted this for ages!

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)