Add ide-assist: generate_impl_trait for generate_impl

[A4-Tacks]

Adds this trait impl for a type.

trait $0Foo {
    fn foo(&self) -> i32;
}

->

trait Foo {
    fn foo(&self) -> i32;
}

impl Foo for ${1:_} {
    $0fn foo(&self) -> i32 {
        todo!()
    }
}

[A4-Tacks]

r? @Veykril

[ShoyuVanilla]

Besides tabstop, other things look good