feat: add fmtstr::quoted

[asterite]

Description

Problem

Reviving the first commit in #5942

Summary

This adds a way to turn a fmtstr into a Quoted value that holds a string token. Otherwise there's no way to create string tokens by interpolating dymamic (well, at compile-time) values.

This PR also fixes an issue with how string tokens are displayed. Previously they wouldn't include the double quotes, and special characters weren't escaped.

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to Brillig bytecode sizes

Generated at commit: 009497b99328d3b27fc3ad717e5dc9c7f8d35157, compared to commit: fc5bb025d7df901050af1d8ad6ebb9283faf641f

There are no changes in circuit sizes

[github-actions]

🚀 Deployed on https://66e319bd7e5e443a56f5ad9e--noir-docs.netlify.app

[github-actions]

FYI @noir-lang/developerrelations on Noir doc changes.

[jfecher]

Quoting & unquoting a format string will already turn it into a regular string so I don't think we need this method.

We could possibly instead add a helper method:

fn as_quoted_str(self) -> Quoted {
    let s = std::meta::unquote!(quote { $self });
    quote { $s }
}

[asterite]

We could possibly instead add a helper method

Would that be a method of fmtstr?

[asterite]

I'll split the "Correctly print string tokens" commit into a separate PR in the meantime...