Make TypedString preserve quote style

[graup]

Fixes #1673. This is a breaking change.

TypedString contained a String without any knowledge of the used quote style. The parser used parse_literal_string to construct this, which doesn't support any quote styles other than single or double quotes. Namely, it doesn't support triple quotes from BigQuery, causing the issue reported in #1673. Additionally, it doesn't round-trip properly, always formatting its string using single quotes.

I think the most proper fix is to have TypedString contain a Value instead, similar to IntroducedString and others. This gives us immediate support for other quote styles and fixes the formatting to make it roundtrippable.

This is a breaking change but should be an easy fix in users' codebases, just (un)wrapping the value. Migration path:

1. When constructing an AST node

Expr::TypedString {
    data_type: DataType::JSON,
--  value: r#"{"class" : {"students" : [{"name" : "Jane"}]}}"#.to_string()
++  value: Value::SingleQuotedString(
++      r#"{"class" : {"students" : [{"name" : "Jane"}]}}"#.to_string()
++  )
},],

2. When using AST parser results

if let Expr::TypedString { data_type, value } = expr {
--  let string_value: String = value;
++  let string_value: String = value.into_string().unwrap();
}

For convenience, I have added a method into_string -> Option(String) to Value to get the underlying string value.

[graup]

Thanks @iffyio, appreciate your suggestions. I've changed the method to into_string -> Option(String). Also updated the PR description.

[PrettyWood]

Thank you so much for working on that 🙏 We have PRQL/prql#5099 on prql side. Very much appreciated

[iffyio]

LGTM! Thanks @graup!
cc @alamb

[alamb]

Werd! The PR / code 🚂 keeps on running. Thanks again @iffyio for all you do to keep this repo moving forward