`AST::LiteralExpr` should differentiate between strings and raw strings

[CohenArthur]

Currently parsing r#"gccrs"# works, but it returns a LiteralExpr with LiteralType of Literal::STRING, but we have a Literal::BYTE_STRING variant. It would be useful to have a Literal::RAW_STRING variant too, and we do need it for format_args!()

[AnshVM]

Hey, I would like to work on this issue