feat: extension `Merge match arms`

Add assist, `Merge all match arms`

## Example

```rust
fn main() {
    let n = 2;
    $0match n {
        0 => 1,
        1 => 1,
        2 => 2,
        3 => 2,
        4 => 3,
        5 => 3,
        _ => 0,
    };
}
```
->
```rust
fn main() {
    let n = 2;
    match n {
        0 | 1 => 1,
        2 | 3 => 2,
        4 | 5 => 3,
        _ => 0,
    };
}
```