Fix postfix completion indentation compensation

[A4-Tacks]

Editor adds the current indentation to the content of the code snippet

This PR dedentation is used to offset the editor snippet indentation

Example

fn foo(x: Option<i32>, y: Option<i32>) {
    let _f = || {
        x
            .and(y)
            .map(|it| it+2)
            .$0
    };
}

Before this PR

fn foo(x: Option<i32>, y: Option<i32>) {
    let _f = || {
        let $0 = x
                        .and(y)
                        .map(|it| it+2);
    };
}

After this PR

fn foo(x: Option<i32>, y: Option<i32>) {
    let _f = || {
        let $0 = x
            .and(y)
            .map(|it| it+2);
    };
}