Add ide-assist: convert_to_format_string

[A4-Tacks]

This assist overlaps with the postfix completion .format,
difference is that this assist only converts one curly, and all other curls will be escaped

Example

fn foo() {
    let n = 2;
    let s = "n: {n$0}";
}

->

fn foo() {
    let n = 2;
    let s = format!("n: {n}");
}