Add ide-assist: convert_attr_cfg_to_if

[A4-Tacks]

Close #20274

Convert #[cfg(...)] {} to if cfg!(...) {}.

fn foo() {
    $0#[cfg(feature = "foo")]
    {
        let _x = 2;
    }
}

->

fn foo() {
    if cfg!(feature = "foo") {
        let _x = 2;
    }
}

Convert if cfg!(...) {} to #[cfg(...)] {}.

fn foo() {
    $0if cfg!(feature = "foo") {
        let _x = 2;
    }
}

->

fn foo() {
    #[cfg(feature = "foo")]
    {
        let _x = 2;
    }
}