Migrate unwrap_block assist to use SyntaxEditor

[A4-Tacks]

• Fix invalid match in let-stmt
• Fix multiple statements loses indent

Example

fn main() {
    let value = match rel_path {
        Ok(rel_path) => {$0
            let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
            Some((*id, rel_path))
        }
        Err(_) => None,
    };
}

Before this PR

fn main() {
    let value = let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
    let value = Some((*id, rel_path));
}

After this PR

fn main() {
    let rel_path = RelativePathBuf::from_path(rel_path).ok()?;
    let value = Some((*id, rel_path));
}

---

fn main() {
    let mut a = {$0
        1;
        2;
        3
    };
}

Before this PR

fn main() {
    1;
2;
    let mut a = 3;
}

After this PR

fn main() -> i32 {
    1;
    2;
    let mut a = 3;
}