Add partial selection for merge_imports

[A4-Tacks]

Example

Input:

use std::fmt::Error;
$0use std::fmt::Display;
use std::fmt::Debug;
use std::fmt::Write;
use$0 std::fmt::Result;

Before this PR:

use std::fmt::Error;
use std::fmt::{Debug, Display, Write};
use std::fmt::Result;

After this PR:

use std::fmt::Error;
use std::fmt::{Debug, Display, Result, Write};