Auto merge of #12452 - iDawer:assist.merge_selected_imports, r=Veykril

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

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