fix: editor postfix dedent uses current cursor

[A4-Tacks]

Fixup #21324

I realized that the editor indentation is based on the current cursor position, not the position after completion

Example

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

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);
    };
}