Extract to function/variable unescapes '\\'

[RagnarGrootKoerkamp]

This is similar to #11897, but for extracting to function/variable instead of snippet completion.

rust-analyzer version: rust-analyzer 1 (ba28e19 2022-10-29)

rustc version: rustc 1.62.0-nightly (30f386087 2022-05-05)

relevant settings: None, I think

Example:

fn main(){
    let a = "\\ $0";
}

selecting the enther let a line and doing extract to variable results in

fn main(){
    let var_name = &"\ ";
    let a = var_name;
}

I.e., both the \\ and the $0 are replaced wrongly.

[lnicola]

Fixed in #16467