feat: Add hex escape

[vanillajonathan]

Add hex escape inside strings such as "Hello \x61".

[max-sixty]

Great to see you're working on the rust code @vanillajonathan ! Nice work.

Could we add a test for this? You can copy one from just below here:

prql/crates/prql-parser/src/lexer.rs

Line 475 in ee71cd7

assert_snapshot!(quoted_string(false).parse(r#"'aoeu'"#).unwrap(), @"aoeu");

. Lmk if you need any guidance...

[vanillajonathan]

Thanks!

I have added a test now.

[max-sixty]

Excellent! Congrats on a nice rust PR...

[vanillajonathan]

Thanks!