feature: `Merge imports` assist can merge multiple selected imports.
The selected imports have to have a common prefix in paths.
Select imports or use trees to merge:
```rust
$0use std::fmt::Display;
use std::fmt::Debug;
use std::fmt::Write;$0
```
Apply `Merge imports`:
```rust
use std::fmt::{Display, Debug, Write};
```
Closes#12426
bors
ea15e1000f