Is your feature request related to a problem? Please describe.
Related to Fantomas bug fsprojects/fantomas#560.
In short, the AST does not keep track of the @ character before a string.

let s = @"\"

Creates

SynConst.String("\", tmp.fsx (1,8--1,12) IsSynthetic=false), tmp.fsx (1,8--1,12) IsSynthetic=false)

Describe the solution you'd like
Can we extend SynConst.String to indicate the leading @ was present?

Describe alternatives you've considered
In Fantomas, we currently detect the @ via the tokens.

Additional context
If approved, I'm willing to try and implement this myself.