Fix string escaping

[aljazerzen]

The is a bug in string escaping code.

Reproduction:

fn main() {
    use sqlformat::*;

    let res = format("''some text ending with escaped quote''", &QueryParams::None, FormatOptions::default());
    println!("{res}");
}

prints:

' some text ending with escaped quote ' ''

but should print:

'some text ending with escaped quote'''

It's a small bug in tokenizer::take_till_escaping, which I fixed and added a test to cover this case.

[aljazerzen]

PS: we use this crate at PRQL and this is how we found the bug.

[shssoichiro]

Thanks! Much appreciated! (This makes me wonder if the Javascript library this is ported from has the same bug 🤔 )